Spent over 100 hours in
Resident Evil 5's The Mercenaries Reunion. I haven't played much of RE3's Mercs, but I have played tons of RE4, RE5, RE6 and REMercs3D. In my opinion, the Mercs modes (excluding RE3) are ranked like this:
RE5Gold > RE6 > REMercs3D > RE4 = RE5
I rank them like that because I find RE5Gold--The Mercenaries Reunion--to be the most playable (and casually accepting), whereas RE5--The Mercenaries--and RE4 both take huge degrees of skill to perform casually decent (meaning 4 stars for RE4, and a basic SS rank for any stage in RE5). I also highly prefer RE5Gold's score system (the combo-based system, not the time based one) to any other RE Mercenaries that I have ever played. Reunion might be an easy mode, but it's still the easiest to get into since every character is basically OP (though some characters perform infinitely better than others if you're a pro player), and it also has the best characters in their fullest capabilities: Barry and Rebecca. <3
You just need to get the time bonus + skill and it gives you up to 8 minutes if you hit them all. Plus you need ALOT of melee kills for the 5 second bonus.
Not even melee kills; you should rely on counters for the +10 seconds.
EDIT: The game with the best unlockables? Tie between 6 and Mercs3D. Skills to improve your overall gameplay? Yes please!