There's not much I can say that hasn't already been said about RE4. Like Chrono Trigger, it's one of those games where everything perfectly fits into place. The core shooting and melee mechanics aren't as fleshed out as 5's, and the Mercenaries mode isn't as good either, but the main campaign's level design is definitely much better.
I want to recommend the GameCube version specifically. The US/JP (not PAL) GC versions are balanced a little differently from later ports. Most of your weapons (including the knife) do less damage, you get less money over the course of a playthrough, and enemies are both more aggressive and more numerous. It's a more subtle difference than I may make it appear - this isn't like Mario 3 or Castlevania 3 - but you can still feel it nonetheless. It encourages more aggressive play utilising the melee mechanics. While the original GC version lacks Separate Ways, I still consider it to be the definitive version because of the superior balancing.
I want to recommend the GameCube version specifically. The US/JP (not PAL) GC versions are balanced a little differently from later ports. Most of your weapons (including the knife) do less damage, you get less money over the course of a playthrough, and enemies are both more aggressive and more numerous. It's a more subtle difference than I may make it appear - this isn't like Mario 3 or Castlevania 3 - but you can still feel it nonetheless. It encourages more aggressive play utilising the melee mechanics. While the original GC version lacks Separate Ways, I still consider it to be the definitive version because of the superior balancing.