Well, that depends on the gun type. My opinion, as follows:
Assault rifle: Definately the Steyr AUG, but the Diemaco C7 is also quite good(even better than M16A2!), especially with fitted optics.
Submachine gun: H&K MP5's a great one, but there's nothing like a good ol' Thompson M1A1. Ever tried the FN P90? Light, compact, 50rd clip, great for close-combat! Dutch Special Forces use them.
Machine gun: At first, I would say the M60E3, but I'm particulary fond of the FN Minimi (or M249 SAW, is the same).
Sniper rifle: I've tried the L96A1. Great weapon, but has a recoil that kicks like a mule

And of course the M40A3. For semi-auto freaks, there's only one true option: H&K PSG-1 (nice option on this weapons is, it can be equipped with a silencer)
Pistol: IMI Desert Eagle!!! Especially the .50AE! For guys with somewhat weaker wrists: Glock-17. Really light and very accurate.
Revolver: Don't have much use for those, because pistols have so many more advantages, but I really like the .454 Casull.
Shotgun: How 'bout the Ithaca-37?