Naaa
Megas might be the best gimmick anime wise and coolness wise, but in terms of battle mechanics all the other battle gimmicks are actually much better suited for the metagame. Megas was unfair as only few mons could use it, the rest of the gimmicks are much more advantageous purely from a battle mechanics perspective
The issue with megas is that they gave them to a bunch of Pokemon that did not need the buff. Garchomp, Scizor, Salamence, Gyarados, Tyranitar, Mewtwo, and Rayquaza were already incredibly strong Pokemon. In fact, regular Garchomp, Tyranitar, Scizor, and Gyarados were often used in their regular forms when megas were still available just because the stat boost wasn't necessary when compared to other items that they could use. I remember thinking that my Tangrowth could wall a Gyarados only to get blown up by Supersonic Skystrike. I get that they are fan favorites, but it was seriously too much on those guys.
On the other hand, Pokemon like Beedrill, Pidgeot, Mawile, Sableye, Altaria and more really appreciated having megas since they fell off in viability over the years or were garbage from the start. They never even considered using other items since their regular forms never had a chance at competing in the big leagues. Mega evolution Beedrill turned from a useless Route 1 bug into a demon.
The problem with the other gimmicks is that everything gets them. It's like why would I waste Dynamax on my Unfezant when Gyarados is stronger and has better abilities? They keep the good Pokemon good, and the bad Pokemon remain on the same level in comparison. If megas were given to the right Pokemon instead of the broken stuff, then it would be a lot more fair. Z-moves are a little better in this regard since you could do cool tricks with certain moves, like Z-Conversion Porygon-Z.
Tera is cool, since there are a million strats you could pull off with all the types, but the stat boost from Mega Evolution is still much better. For example, Houndoom is still gonna be frail and (relatively) slow even with Tera. It would rather just go Super Saiyan and get the stat boosts it needs to outspeed opponents and deal some more damage.