All the Mega Evolutions the compiled Pokémon Showdown data set carries — 91 forms across 85 species, including the Pokémon Champions additions. 74 of them are legal in Regulation M-B and carry measured ladder usage; the rest are listed with a dash because there is no legal play to measure.
Usage is Smogon's published figure for the Regulation M-B VGC ladder. Sorting is by that figure, so the top of the list is what you will actually meet.
Pokémon Champions has no Terastallization. Mega Evolution replaced it as the mechanic a team is built around, which is why a Mega slot is now the first decision rather than the last. Each entry below has the stat change, the full type board, and — where the ladder has enough games — the set people actually run.