Funniest- The Ice/Freezing gun in Duke Nukem 3d... However, a close runner-up is the shrinking gun where you'd run up and step on the guy to kill him afterwards
Best "real" gun - Kar98 from DoD... Oooooh baby that handles nicely
Coolest - Lightsaber from Jedi Outkast2 (never played the 1st one). It was all right in single player, but when I played MP and you could do saber-only duels with people... That just brought a whole new level of deathmatch to me!