For me:
M1 toss was the best mechanic of the series.
M2 was hilarious but outstayed its welcome (obviously that'll fix itself as dps escalates).
M3 was okay.
M4's slow reveal of the clones has readability issues to me. I have to kinda squint at them.
All four, especially M4 were a huge step up in terms of cinema.

I still don't think any of them are as hard as P2/P3 were at launch. By hard I mean my anecdotal experience of how reliably they could wipe groups early on, though I think I managed to see a wipe at least once of each (so they're harder than Eric, I guess lol).