I really don't like taking a weapon in higher regard than another, but each weapon does have it's pros and cons.
Fist- Useless, w/Berserk okay I guess, but not against projectile foes.
Chainsaw- A tiny bit overrated, however, it can be used to go up to a single enemy and shake em up a bit so they have no time to attack.
Shotgun- Pretty good to use when just messing around and sniping, and using your highly developed sidestepping skills (no wait, that's just me). But it becomes obsolete a little later.
Super Shotgun- Awesome. Can pick off distant enemies, and has no trouble with the bigger demons. Only con has to be reload time, and it has a big usage of a small capacity of shells.
Chaingun- The Chaingun is always usually your last resort when you run out of rockets or cells. I personally love this weapon. The fact of just goring those enemies to death with the chaingun is awesome. The con of the chaingun is that when your low on bullets and you go at a big enemy it kinda sucks because bullets are the weakest out of the ammunition.
Rocket Launcher- I'd say the RL is the most fun weapon. Just picking off enemies at a distance is fun. The con? Recoil and lack of ammo.
Plasma Launcher- Rapid gun with the best ammo in the game? I love it. I use more cells with this than the BFG because I only use the BFG on the big enemies as a first round beating to weaken them up a bit. But against anything but the Masterminds and Cybers, the Plasma Launcher is the best weapon.
BFG- Uses to much cells, dude.