RE4 is most excellent, the graphics are chock-full of detail, the enemies are varied enough to keep things interesting, there's a good amount of treasure hunting to do that actually really helps a lot, the weapons are cool and when you beat it, you can start over again with all your weapons, so you can keep playing to upgrade your weapons and get the new ones. There's even a special weapon that becomes available after you beat it the first time.. and it is very powerful, is all I will say. From what I know, aside from the normal weapons, there's about 2-3 secret weapons to get.
And as Alea said: Thank God for them finally getting away from all that Umbrella bull****, it was totally getting old for me. RE4 was my first RE game, but I've seen and heard a lot from the older RE games because a cousin of mine is an RE fan, and RE4 is the first one that actually appealed to me. Even if you were to prefer RE4's storyline to go on with Umbrella instead of the current story line, the game itself is really really great and Leon moves and acts much more dynamic. I mean, now you can even hop over fences that are knee-high, jump through windows from the second story, all those kinds of things. Hopping a small fence might not SOUND like much of an achievement, but you couldn't do that in the previous games, and it sure as Hell beats having to walk around it or find a small gate to get through it.