Tyranitar: A ---> A-
Mega Gardevoir: A ---> A+
Mega Alakazam: A- ---> B+
Tyranitar: I'm kind of on the fence here, but leaning towards A. Tyranitar has really fallen from grace as the metagame has evolved, but he's still one of the most used mons for a reason. Hippowdon really is kind of outclassed by Landorus-T nowadays, so this dude is the main thing to support Excadrill. Of course mold breaker scarf and stuff is viable, but sand rush Excadrill is the most threatening Excadrill by a lot. Tyranitar can be a decent check to stuff like CharX and Talonflame and has a very wide movepool, making him a good supportive stealth rock setter. Recently, new developments like HP fighting Latios are really a pain though, and I think this is why he's no longer A+, but A is still good. Most people forget it can do other stuff too. I have one team that's volturn and it's built around getting in CB Tyranitar and Medicham- it works pretty well for trapping Latis without HP fight, which means rocks are most definitely permanent. Scarf is also pretty awesome, takes out Latis and Gengar. With these gone, stuff like Keldeo and Charizard-Y can go to town on the opposing team. BW keldtar is still excellent if you can check Clefable, and CharTar is also great. I don't have a ton to say here, but basically it supports Excadrill, supports Keldeo and Charizard-Y (among other special sweepers who appreciate pursuit support.) and has a really awesome movepool. Is it amazing? No, it has a lot of common weaknesses. However, I think empowering sand offense is enough to verify it's placement in the A tier, Excadrill kind of needs him. Tyranitar + Latios + Ground + Ferrothorn/Heatran + Water + Mega is a really common and solid team, and cannot function without Tyranitar.
Mega Gardevoir: This mon is interesting, and I think it should move up. The main issue is it doesn't have any physical bulk, and you need to get it in on certain mons to make it work well. If you do, it will kill things. Due to the typing and SpD, it's pretty hard to revenge kill for offense. Nowadays, stall can beat it (Doublade...) but it still puts in work there. Along with Pinsir and CharX, this is one of the few megas that has immediate jaw dropping power. Hyper voice is nuts, and all that can really take it is steels and fire types, which are downed by fighting and psychic moves respectively. It has almost no counters, and gets kills vs. balance pretty much every game if played right. The main flaw is that it has awful defense, so it is easy to revenge kill. It can take one special hit from almost anything, which means it can kill them, usually in one hit. Sorry for repetition, but once it comes in on something like Ferrothorn, very little can take its powerful hits. The speed tier is not amazing, but it is often good enough. In summary, little switches in, very few counters, bad physical defense, medicore speed. I'm not sure if it deserves A+, but I think that's enough, it's definitely as good as some other pokemon up there.
Mega Alakazam: Yep, completely agree here. I never really understood why Alakazam jumped all of a sudden, it's not that amazing. To be blunt, Alakazam is fast and powerful. It's stupidly fast, I will undeniably deign it that (it outspeed everything boosted.) However, it is not that powerful. It cannot OHKO uninvested Charizard and similar mons. With the right coverage it can 2HKO almost anything, but it really can't take any hits. Before he can even revenge kill or get off an encore, he is susceptible to priority. Unlike fast mons like Aerodactyl, Alakazam can't take any priority, and will fall to Pinsir, Dragonite, etc. This really bothers me, and makes his role as a revenge killer situational. Thundurus has a bit of bulk to taje some priority, and why he might not kill things, he can at least make them useless. Encore is only really useful against set up sweepers if Alakazam comes in the turn the set up move is used, which can only be safely facilitated through U-Turn or Volt Switch. It can also be a hinderance to support mons using stuff like stealth rock. Alakazam basically needs Psychic/Focus Blast/Shadow Ball to work well, and encore is one of its main selling points, necessary in the fourth slot. Without shadow ball... Latios can defog and roost, before finishing him off, Psychic is STAB, and Focus Blast is for steels. Even with this, he can't 2HKO everything, or OHKO where his moves are super effective. So this is my summary: very fast, but not strong enough to always revenge kill, weak to priority, and not powerful enough to sweep stuff. Doesn't sound like a B+ mon, but there's one thing I left out: Trace. Alakazam uses this do goddamn well it's ridiculous. While Landorus has dropped in usage and viability, a Mega Alakazam revenging it is ridiculous, and offense without strong priority or a mon bulky enough to take a sheer force boosted move are in serious trouble- it's pretty much always guaranteed at least the kill on Landorus or whatever it switches into. Next is protean from Greninja, which is almost just as bad, but the nuclear psychics of sheer force are not present. Zam can switch in to Heatran and absorb what's often its only attack. While those are the best examples, other abilities have uses too. Zam has some flaws, but this introduces a threatening dynamic which can make him worth the mega slot, but not always. Hence, I think B+ is appropriate.
Mega Gardevoir: A ---> A+
Mega Alakazam: A- ---> B+
Tyranitar: I'm kind of on the fence here, but leaning towards A. Tyranitar has really fallen from grace as the metagame has evolved, but he's still one of the most used mons for a reason. Hippowdon really is kind of outclassed by Landorus-T nowadays, so this dude is the main thing to support Excadrill. Of course mold breaker scarf and stuff is viable, but sand rush Excadrill is the most threatening Excadrill by a lot. Tyranitar can be a decent check to stuff like CharX and Talonflame and has a very wide movepool, making him a good supportive stealth rock setter. Recently, new developments like HP fighting Latios are really a pain though, and I think this is why he's no longer A+, but A is still good. Most people forget it can do other stuff too. I have one team that's volturn and it's built around getting in CB Tyranitar and Medicham- it works pretty well for trapping Latis without HP fight, which means rocks are most definitely permanent. Scarf is also pretty awesome, takes out Latis and Gengar. With these gone, stuff like Keldeo and Charizard-Y can go to town on the opposing team. BW keldtar is still excellent if you can check Clefable, and CharTar is also great. I don't have a ton to say here, but basically it supports Excadrill, supports Keldeo and Charizard-Y (among other special sweepers who appreciate pursuit support.) and has a really awesome movepool. Is it amazing? No, it has a lot of common weaknesses. However, I think empowering sand offense is enough to verify it's placement in the A tier, Excadrill kind of needs him. Tyranitar + Latios + Ground + Ferrothorn/Heatran + Water + Mega is a really common and solid team, and cannot function without Tyranitar.
Mega Gardevoir: This mon is interesting, and I think it should move up. The main issue is it doesn't have any physical bulk, and you need to get it in on certain mons to make it work well. If you do, it will kill things. Due to the typing and SpD, it's pretty hard to revenge kill for offense. Nowadays, stall can beat it (Doublade...) but it still puts in work there. Along with Pinsir and CharX, this is one of the few megas that has immediate jaw dropping power. Hyper voice is nuts, and all that can really take it is steels and fire types, which are downed by fighting and psychic moves respectively. It has almost no counters, and gets kills vs. balance pretty much every game if played right. The main flaw is that it has awful defense, so it is easy to revenge kill. It can take one special hit from almost anything, which means it can kill them, usually in one hit. Sorry for repetition, but once it comes in on something like Ferrothorn, very little can take its powerful hits. The speed tier is not amazing, but it is often good enough. In summary, little switches in, very few counters, bad physical defense, medicore speed. I'm not sure if it deserves A+, but I think that's enough, it's definitely as good as some other pokemon up there.
Mega Alakazam: Yep, completely agree here. I never really understood why Alakazam jumped all of a sudden, it's not that amazing. To be blunt, Alakazam is fast and powerful. It's stupidly fast, I will undeniably deign it that (it outspeed everything boosted.) However, it is not that powerful. It cannot OHKO uninvested Charizard and similar mons. With the right coverage it can 2HKO almost anything, but it really can't take any hits. Before he can even revenge kill or get off an encore, he is susceptible to priority. Unlike fast mons like Aerodactyl, Alakazam can't take any priority, and will fall to Pinsir, Dragonite, etc. This really bothers me, and makes his role as a revenge killer situational. Thundurus has a bit of bulk to taje some priority, and why he might not kill things, he can at least make them useless. Encore is only really useful against set up sweepers if Alakazam comes in the turn the set up move is used, which can only be safely facilitated through U-Turn or Volt Switch. It can also be a hinderance to support mons using stuff like stealth rock. Alakazam basically needs Psychic/Focus Blast/Shadow Ball to work well, and encore is one of its main selling points, necessary in the fourth slot. Without shadow ball... Latios can defog and roost, before finishing him off, Psychic is STAB, and Focus Blast is for steels. Even with this, he can't 2HKO everything, or OHKO where his moves are super effective. So this is my summary: very fast, but not strong enough to always revenge kill, weak to priority, and not powerful enough to sweep stuff. Doesn't sound like a B+ mon, but there's one thing I left out: Trace. Alakazam uses this do goddamn well it's ridiculous. While Landorus has dropped in usage and viability, a Mega Alakazam revenging it is ridiculous, and offense without strong priority or a mon bulky enough to take a sheer force boosted move are in serious trouble- it's pretty much always guaranteed at least the kill on Landorus or whatever it switches into. Next is protean from Greninja, which is almost just as bad, but the nuclear psychics of sheer force are not present. Zam can switch in to Heatran and absorb what's often its only attack. While those are the best examples, other abilities have uses too. Zam has some flaws, but this introduces a threatening dynamic which can make him worth the mega slot, but not always. Hence, I think B+ is appropriate.