Let's see...
Blaster, yes, it has unlimited ammo and is one of the most powerful weapons in the game when powered up and maximum fastfire. (unless you cheat ofcourse, then the laser shield is best)

Bouncer, yes, it can be used to shoot downwards to kill baddies you can't reach and it travels furthest upwards, allowing it to shoot down floating objects you cannot reach.

Freezer, yes, handy for freezing springs and you can kill baddies with spectaculair fashion by running into them while frozen.

Seekers, YES, very handy for killing flying baddies and bosses.

RF Missiles, yes, good for destroying countdown blocks and loads of baddies on one spot.

Toaster, yes, has continuous flame and can seriously injure bosses and has a shot delayment that can kill upcoming baddies.

TNT, yes, can blow up loads of blocks in one shot and many baddies in one shot.

Pepper spray, no, has nearly the same function as the blaster, except that it fires diagonally when you suddenly try to shoot upwards.

Electro blaster, no, is extremely weak, even when powered up and has no special function except going through walls, which is not very usefull.

Trafton, YES!, wha?!?

This is my opinions about the weapons in SP mode. I have never played MP on the internet before, so I don't know.