As a competitive enjoyer, Terastalisation is by and far the best gimmick. It allows for a lot of both offensive and defensive flexibility, as well as plenty of options. If a certain Pokemon is proving to be problematic in the current season, swapping tera type to be a favourable matchup is well within reason. Likewise, patching up a weakness, giving a Pokemon the oomph it needs to secure a KO, even ensuring it can survive for one more turn, all of these options are make or break in any VGC match.
However, as far as "rule of cool" goes, mega evolution is easily the popular one. It's no surprise that TPC have brought it back, both as a standalone gimmick in a new game, and through the competitive scene in an isolated environment in Champions. That way, other generational gimmicks won't clash with each other in new titles, while the benefits of them can still be experienced in what is likely going to be where VGC takes place from here on.
