The most funnest weapon is always the Rocket Launcher. I mainly voted for it because I played multi-player doom more then single-player when I first got Doom. I don't care about winning, I only cared about having fun (while winning). It was always fun for me to watch my enemies get blown apart as they're running away. Yeah, sure, run away. Just expect a rocket up your ass the second you turn around! If it was possible to make it so second button shoot a secondary weapon of your choice, mine would always be the rocket launcher.
I guess the second best weapon would be the shotgun or the chaingun. Though it uses too much ammo too quickly, the chaingun is nice because it usually keeps the enemy you're shooting unable to fire back while you're still shooting them.