I think the most ridiculous guns were the ones in Turok. You had the ones that basically stopped the game for half a minute while the explosions happened, and there was also that gun with the homing brain drill that you could not avoid once it was locked on.
If we're talking ridiculous, let's talk People Can Fly. Painkiller's weapons were like a list of the most powerful weapons from half a dozen other games.
For me, I'll take a little bit of a different angle and say the Blaster Launcher from the original XCOM. It was a rocket launcher that fired a projectile that could be commanded to follow a path around corners, up stairs and into the heart of the enemy base. Once it got there, it destroyed half a room, while your soldiers were smoking cigars halfway across the map.