It seems like my rankings might qualify for the unpopular opinions thread. My views are primarily shaped by designs, the lore/depiction/use in the games and anime and how enjoyable/interesting the mechanics have been as a spectator of VGC tournaments. I don't care about Smogon's formats, so my rankings are not shaped at all by those.
1. Dynamax
Although I initially disliked the concept of giant Pokemon, especially after the ridiculous giant torterra in the Detective Pikachu movie, the in-universe lore that it is some sort of hard-light-holographic projection is acceptable enough for alleviating that gripe (according to a single sign late game + an interview). It would have been nice if they focused on that aspect at all in the anime, but whatever, I'll take it. With that issue resolved, the reason dynamax makes the top of my list is
i) I think it actually gives the best feeling of a really exciting energetic battle---like if you were actually watching a Pokemon match in a stadium and one of the mons turned huge, that would be way more exciting than the other gimmicks. This was conveyed pretty well in the anime, with several david vs. goliath-esque battles.
ii) In VGC it did make for some really cool strategies, like the prankster trick room setup, and there were some exciting moments in tournaments, like the players cup where Wolfe dynamaxed his Urshifu-R instead of Coalossal, to the surprise of viewers and casters (from Wolfe's point of view he knew his Coalossal was slower than his opponent's). With the 3-turn limit it also didn't feel like the matches were purely decided by whoever managed to KO the other dynamax mon first, and there was a lot of variability and strategy as to when players dynamaxed their mons. The fact that any mon could dynamax led to more interesting choices than with Megas
iii) Hot take apparently, but I think the Gigantamax designs are overall better than the Mega designs. I'll say more about that when I get to Megas, but essentially I think the Gigantamax forms better captured the essence of the base designs while making them look and feel more powerful. In several cases the Gigantamaxes were enormous improvements over the base design too (cough Drednaw Coalossal cough).
2. Terastallization
This is a tentative placing since we're not through the generation yet; the DLC could bring some additional twists on terastallization, and the gimmick has only had one depiction in the anime so far.
This said, the reason I'm placing this 2nd for now is largely based on the VGC tournaments so far. Terastallization in VGC has been super interesting to watch, and a lot of the decisions have been largely for defensive purposes rather than just pumping up hyper offense. The fact that all official VGC tournaments are open team-sheet helps a lot with this, I think, because it prevents the matches from being riddled with too many surprise teras. While a player might not always guess correctly when a Pokemon will tera or which one will tera, they at least know what the options are and can go for plays like doubling-up on a Pokemon with moves that will hit the original typing or the tera typing (or ideally a switch in).
I think in-universe it would feel less exciting when a Pokemon terastallizes compared to dynamax, which helps place tera at the number 2 slot. I think if the DLC happens to bring something like tera forms, it might only solidify tera's position at number 2 on my list, rather than bump it up past dynamax, but that would really depend on what any hypothetical tera forms would do, and how many mons they're distributed to. e.g., if it just had something to do with the masks in part 1, that might be pretty meh. At any rate, we'll have to wait and see.
3. Z-Moves
It was hard to pick between Z-moves and Megas, because on the one hand I like form changes, which Z-moves did not offer, but on the other hand I have a soft-spot for Z-Moves because of Marshadow's move, and because I think Z-Moves came off really well in the anime, and those are the factors that tipped it to 3rd place.
In particular, the depiction of Z-Moves in the anime (and partly in the games) gave me the impression of a connection between the trainers and the Pokemon more than Megas did. While there were several episodes focused on trainers working together with their Pokemon, I don't know, it just felt like Z-Moves involved the trainers sharing their energy with their Pokemon to pull off a powerful attack.
The clash between Pikachu and Tapu Koko's Z-Moves, as well as later Pikachu vs. Gigantamax Charizard are also now very iconic depictions of the mechanic (both of those examples also tying into the 'excitement' of a powerful move being used against a huge opponent).
In terms of VGC, Z-Moves weren't quite as exciting, as while they could make for some impressive moments in games, they often didn't feel as exciting as they could be pretty inconsistent: you either nuked a mon or they protected to only take pitiful damage, or worse they switched in a mon with an immunity and the big cinematic attack did nothing. It would also have been cool if a few mons other than just Necrozma had associated form changes from using Z-Moves (which even Necrozma didn't really have -- it was really just a mega that unlocked a Z-Move after use).
4. Megas
Okay, hear me out here. I don't hate megas, I just don't think they have held up as well on the various fronts of the franchise. There are several reasons they have ended up in the bottom slot:
i) Designs. As I said earlier, I think Gigantamax designs were overall better than Megas. This point may be biased by the absolute travesty that is Mega Blastoise compared to the amazing Gigantamax Blastoise. I just hate Mega-Blastoise's chin so much. Why did they give it that chin? The forehead doesn't do it any favors either. In terms of cannons I like what they did with Gmax-Blastoise way more. It looks more organic, whereas Mega Blastoise looks too clunky, kind of like Transformers kibble.
Similarly, while I like Charizard X better than Gmax-Zard, the Gmax beats Charizard-Y, and G-Max Venu beats Mega Venu. I like both Mega Gengar and Gmax Gengar, but I think I like Gmax Gengar's design better. I think Mega Mewtwo-X was over designed, and while I like Mega Mewtwo Y ok, I think it is still a downgrade from regular Mewtwo.
Mega Banette and Mega Mawile are amazing, though.
I think that's the crux of what I like about the Gmax designs over megas: they generally feel more organic. Megas generally made things too smooth.
ii) In terms of watching VGC, memory is faded by now and I watched less VGC then than now, but I have the impression that Megas were a lot more centralizing in the format because only a select number of Pokemon had them. While it did bump some previously forgettable mons into the spotlight it was also just given to a lot of already good mons that made the opportunity cost of using the worse mons too high. Plus, because Megas required the item slot it meant you would less frequently see multiple possible megas on a team, making it a bit less interesting to watch (though that may be a positive to the players!)
iii) Megas were depicted fine in the anime, and the final battle between Mega Zard X and Ash-Greninja was definitely exciting (right up until the end), but overall I don't think Megas have been depicted as well as the other gimmicks. For example, both Megas and Dynamax had episodes in which a non-Mega/Dmax mon had to go up against them, and for Dynamax it was often portrayed as more high-stakes, while for Megas it didn't often feel as tense -- Ash's Dragonite beat Korrina's Mega Lucario (which had been beaten by Pikachu before, I think), Leon's regular Charizard nuked Alain's Mega Charizard, vs. Ash's regular Lucario having to bust out a "Gmax Aura Sphere" to KO Gmax Duraludon and Pikachu enduring several attacks from Gmax Charizard. Granted a lot of this is the writers' choice, but it doesn't help Megas' case.
iv) Pokemon GO's handling of Megas doesn't do the gimmick any favors. When it first came out it left me entirely uninterested in wasting raid passes to grind mega energy (with species-specific mega energy that you needed to spend
each time to mega-evolve the Pokemon, and you needed to go multiple raids to get enough energy). They eventually revamped the system somewhat and you can replenish mega energy without raiding in some ways, making it better, but not enough to remove all traces of the bitter taste from the initial rollout.