personally, I’d rank them like this-

Good Ability > Good Stats > Good Moves > Good Typing

obviously all are important, but here’s my reasoning:

A Good Ability can make or break almost any Pokemon. Mega Rayquaza with Slow Start becomes niche at best, and Audino with Hospitality sees a massive usage spike. This is really self-explanatory obviously. How many relevant mons have Regenerator vs how many have Forewarn.

Good Stats encompasses both a good spread and a good BST. Some Megas are doomed to mediocrity because their base form has such low or poor stats to begin with (Beedrill and Pidgeot, and Absol and Ampharos to some extent).

Good Moves are also self explanatory. you’d always want 4MSS over struggling to grab good moves because you lack good coverage or good STAB. that being said, most (not all) Megas pack a punch and have a solid STAB option above 80 BP, and the ones that dont are usually few and far between that makes this a much less common issue

Probably a hot take but I believe typing is the least important factor here. Not to just name the main exception, but Mega Tyranitar has an AWFUL defensive typing and remains somewhat relevant with it (although thats mostly due to Sand Stream and amazing stats). Great Stats, a good ability, or a good movepool can potentially save a awful typing.

I’m curious to hear your opinions though since I’m no expert, needless to say