Cause, I think, from experience, that it Mega Evolution happens before any other attack. I don't know about ability activation or switching out.
Mega Evolution and Dynamaxing generally occur at the beginning of a turn but after any switching has occurred; however, if a Pokémon is Mega Evolving or rotating in and then using Pursuit on a Pokémon that is switching out, the Mega Evolution or rotation always happens before the Pokémon uses Pursuit (and therefore before the other Pokémon's switch).
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Mega-Evolution does not actually have an official stage of priority;however, it does surpass Stage 5, which is the fastest stage of priority, held only by Helping Hand.
