There's a lot of fierce debate about which Biohazard game is the best, but an often overlooked competition among games in the series is that argument of importance. Importance itself can be measured in multiple ways, including gameplay innovation, plot development, development budget, role in the console wars, graphical fidelity, and so on. A brief look at all of them:

Biohazard 0: Began on the N64 and then moved to GC, was the first Nintendo-only title, first to have two simultaneous protagonists, start of the series in terms of plot, Rebecca/Billy/Marcus

Biohazard: Very first game in the series setting the pace for everything after it, only game in the series to be remade, two different scenarios, Jill/Chris/Barry/Wesker, multiple endings

Biohazard 2: First game with Raccoon City proper, Zapping system, Leon/Claire/Ada/Sherry, received a remarkable N64 port, began as Bio1.5

Biohazard 3: Last Escape: Carlos/Nicholai, dodge, Live Events, ammo mixing, random events, Nemesis, last true game of the 32-bit era

Biohazard CODE: Veronica: First game of the post-32-bit era, Dreamcast, fully rendered in 3D, return of Wesker, re-released with new cutscenes, dual weapon wielding, Steve, the Ashfords, T-Veronica Virus, Antarctica

Biohazard Umbrella Chronicles: Wii, showed unknown plot aspects re: Bio0/1/2/3, playable Wesker, Sergei, first person on-rails, fake Bio3 chapter, destruction of Umbrella, Russia

Biohazard Darkside Chronicles: Krauser, return of T-Veronica, mutated person who isn't crazy, South American setting, Manuela/Javier

Biohazard 4: Full model change, Nintendo exclusivity controversy, European setting, Las Plagas, Wii remote functionality, began as Devil May Cry

Biohazard 5: First game of the HD era, X360/PS3, multiplatform launch, online co-op, Sheva/Josh, Spencer, death of Wesker

From a brand name perspective, Resident Evil 4 everytime. I'm absolutely positive that the series wouldn't be the huge beast it is now without it, hell i'm almost certain the series would've been in a Dino Crisis-like coma shortly after without the changes RE4 made.

From a story perspective, too predictably i'd go with the REmake. Because it was so gracefully able to take an already solid story and add to it, making it so much more darker than it was originally was. Not something you see too often.

I'm gonna say Resident Evil 2, simply because it took the franchise to a new level and was widely recognised as one of the best gaming sequels ever made. Turned Biohazard into a franchise. People, both gamers and journalists alike may lament the current state of the series but everyone still fondly remembers bio 2 and the impact it made.

Alien....Aliens.
Terminator...Terminator 2
Resident Evil...Resident Evil 2.

Scary Movie...Scary Movie 2.

He's not wrong! haha

Gonna have to agree with RE2 here, for reasons TheBatMan stated.

You could arguably say it's RE4/RE5, but I don't care much for the newbies who didn't discover the game series until now, so. :)

Pretentious much? It's perfectly acceptable for people who enjoyed the series' early on to appreciate the new breed of Resident Evil as well.

You're obviously misinterpreting me there.
I merely stated that I don't care for those who had not touched a single RE-game, until the new ones came out. Where did I state that someone who was a fan of the old ones can't enjoy the new ones?
Hell, I'm one of those myself. I really like RE5. It's not a fantastic game but it's very entertaining, and much better than other crap out there.

Your comment comes off as horribly elitist, that's all.

*Shrug* Sorry.
Not meant that way eitherway.

I didn't think it was meant that way, i know you're not like that usually.