Least favorite to favorite is Dynamax, Megas, Z-Moves, and then Terastal. I haven't read the other posts in this thread so forgive me if I'm repeating things people have said a bunch.
Dynamax I used to like because of how you could only use it a limited amount ingame and how it allowed for raids. Now Terastal exists and does all of those far better than Dynamax, which leaves the rest of the mechanic. This mechanic is a complete mess and I don't think I need to explain why. Competitively it was the most absurdly broken thing Game Freak has put out thus far, bad enough for the entire singles playerbase to re-evaluate what makes a mechanic like this cross the line, and Gigantamax forms are very pointless and kind of a waste of the designs, which kinda leaves you with nothing else to like about them. Dynamax Adventures are something you can still hold to them I guess, those are legitimately very fun, but still. I really wish I had more to say but there's just nothing to like here.
Megas are cool to me for not being actively harmful to competitive play like the rest of these, but I still dislike them. A lot of people say they're a nice boost to Pokemon that really needed some attention, but you cannot argue to me that at minimum half of the Pokemon that got Megas actually needed them, either from popularity, already being good Pokemon, or both in some cases. I get the mechanic being given to popular Pokemon since it's supposed to be a marketing gimmick but the amount of already very popular Pokemon that got them is absurd. Stuff like Houndoom, Glalie, and Medicham getting a mega is cool, give me more of those instead of a Gardevoir or a second Charizard. If the point of this mechanic was to give bad Pokemon a boost then they would have done that instead of this.
They also feel very egregiously shoehorned into Pokemon X and Y as games. If I recall correctly literally four trainers in those games have one at all and most obtainable Megas are locked behind postgame for some ungodly reason. For what it's worth though they mostly don't impact competitive play outside of some Pokemon not caring about Knock Off, not being able to have two at once which is kinda annoying for teambuilding sometimes, and the occasional "which Mega are they" deal which almost never comes into play because it's always obvious what Mega (or sometimes even which Charizard form) they have. Mostly net neutral there to me. I think this mechanic gets way too much praise, probably for the designs that in my opinion are almost always downgrades from the base forms.
Z-Moves I think are sometimes pretty fun from an ingame standpoint, but that's about it. When I said that Megas were shoehorned into XY, this is basically the opposite - it's cool that they're prizes for the games' Gym Leader equivalent, the whole dance thing is fun, and I'm not gonna make as big of a deal as I'm sure others will about it but I like the jump from X and Y where now any Pokemon can use them. It's really whatever to me outside of being able to use them from any point in the game now. It is annoying having to go through the whole animation of a Z-Move every battle in order to play optimally but it's not the worst thing in the world.
From a competitive standpoint though, awful mechanic. I sure do love not knowing which of my opponent's Pokemon is going to have a Z-Move and then not knowing which Z-Move they have. I sure do love losing the game because the right Pokemon on their team had a Z-Move to blow past the one answer I had to it. You can predict when your opponent is going to use a Z-Move or what Pokemon has it but there's so many layers to it that always just felt obnoxious to me and made so many Pokemon completely unfun to fight. For what it's worth, some status Z-Moves are cool like Z-Conversion and Z-Haze, but those were never the best way to use them so it's not as big of a positive as it could've been. Either way, the ingame thing is at least a solid reason for me to like the mechanic, which the two before it haven't really had much of for me, so here we are.
And then there's Terastal. I'll say again that I like the finite ingame use thing and the fact that any Pokemon can use it. Raids in this game are also leagues better than anything Dynamax's ever did. Those were just kinda something to do, these are something you can actively build Pokemon around and devise deeper strategy for, which is pretty cool even if it does make them pretty easy. It would be absolutely perfect ingame if it were less of a grind to change a Pokemon's Tera type. The DLC will probably fix this but it shouldn't have to be that way. I don't care about Pokemon games enough to spend that much money on them, but it still is nice to hop onto a seven star raid every once in a while, and I think that's a kind of lame thing to limit behind a paywall.
Competitively, it's...not the worst thing out there? It could be leagues better but I prefer it to Dynamax and Z-Moves. It's still extremely uncompetitive to have random 50/50s where "does the Kingambit tera or do they not." It's also significantly more annoying dealing with more experimental tera types, but otherwise it usually is very feasible to predict when someone is going to tera and much easier to punish when misused. I'd rather it not be here and am certain it could be implemented better, there's a Pet Mod that's actually done this and from watching replays it looks fun to mess around with there, but it's still not great.
That's the best compliment I can give any of these - likeable, but not great. I kinda hate the way video games are to some people nowadays where everything needs some big, flashy gimmick in order to have any purpose. If the deciding factor for whether or not you buy a game in the first place is some new big game-defining feature then you'd definitely not know enough about the game to say if that big flashy gimmick actually makes the game more or less fun to play. They kind of need to impact gameplay in some noticeable way, but it's difficult to do that in a way that doesn't completely tear the game into being unhealthy. Megas did this but I don't have confidence the developers will have any ideas like that again, and I don't have any confidence they'll make something more along the lines of Z-Moves or Terastal that doesn't also end up being pretty unhealthy since they (understandably) don't understand anything beyond surface level competitive play and seem to be focusing on doubles anyways. So here we are now, stuck with them for an indefinite amount of time, with the one mechanic most people enjoy probably being locked away forever and with the rest having more and more shared traits that imply we'll never get something truly good out of it.
