Blaziken ==> A: Salamence's introduction hurts it somewhat but its sweeping ability and raw power are still respectable. Its typing, Speed, and power also make it relatively easy to put into a team IMO - Blaziken being the #5 most used Pokemon says something about that. The occasional Baton Pass can also do wonders. Blaziken can either run mega or not, often substituting in Life Orb if there's another mega on the team (or Lefties/Weakness Policy for BP), giving it some flexibility.
Terrakion ==> B+: Aegislash and Landorus-T existing makes its life super hard, forcing it to Earthquake for any shot of removing it. 108 Speed is also not too special - plenty of things outrun it and can destroy it. It's also a rock type that doesn't resist Flying, which is actually a big deal since Rock has a lot of annoying weaknesses and the Flying resist is a huge reason why you'd have a Rock type on your team.
Tyranitar ==> A-: One of the most versatile Pokemon in the game and does a solid job at checking birds. Its unique set of resistances and high bulk makes it an easy shove in to a lot of teams, and it can really run whatever it wants to cover up the team's issues. It does have a massive number of weaknesses unfortunately, although its massive special bulk does allow it to tank through a lot of them.
Slowbro ==> B+: It's one of the best answers to Blaziken in the game and also makes Swagkey cry thanks to Regenerator. Core Pokemon in a lot of stall teams and is also a fantastic pivot. Checks several high profile threats such as Salamence, Garchomp and Kangaskhan, although it may need to take care switching into them as it can be 2HKOed. That said however it's weak to most Special Attackers and it's also a pretty big Mega Gengar bait.
Mega Blaziken ==> A: See Blaziken pretty much
Mega Swampert ==> B-: Honestly regular Swampert is overall better. Mega Pert doesn't really do a whole lot of damage, isn't particularly bulky (especially with no recovery), and relies too heavily on Rain support to actually do much. And it takes your Mega slot
Mega Tyranitar ==> A-: Scary Dragon Dancer and is an even better bird check than regular TTar. Worth noting that TTar is one of the few DDers who can just straight up cut through Zapdos and Cresselia. It's also super bulky.
Mega Steelix ==> C-: Meh...
OTHER STUFF:
Mega Salamence ==> A+: Mega Mence is an interesting Pokemon. It's an offensive threat that's actually harder to wall in Ubers than in battlespot. Anyways this might be an unpopular opinion. While Salamence's bulk, speed, power, and STAB are top notch, it is still walled by the common walls (Cress/P2), runs into Zapdos and Rotom-W (to an extent) issues, and overall runs into residual damage issues. Basically, it's relatively simple to prepare for and it also doesn't have a variety of options to deal with its counters (look at how many ways Kangaskhan can cheese its counters). Don't get me wrong though, it's still a pretty metagame defining mon, but with all the Talonflames that ran around the metagame, everyone's already prepared dealing with birdspam so it's not really something new. Salamence's power is too focused on its flying STAB that chances are, as long as you resist flying and have decent physical bulk, you can handle Mence fine.
Mega Manectric ==> B+: Decent mon to have vs offensive teams since it usually has the coverage to hit most of them SE and has the speed to outrun so many things in the game (im looking at you Mega Gengar). Can break down a lot of offensive synergies such as Azumarill + Gengar and Salamence + Aegislash. Intimidate/Volt Switch is also pretty nice. That said however it's still a tad reliant on being able to hit things super effectively since its base damages are kinda low.
Starmie ==> B-: For the most part it's just outclassed by Greninja. It can use Rapid Spin which is kind of cute but most of the time you wish that Spin split is something else, especially since SR isn't -that- common.