It's one thing for a weapon to be ultrapowerful, but like others have mentioned in the thread, there's also a question of "style", usefulness, or enjoyability. For instance, the pulse rifle is powerful, but I don't like to use it because it tends to fry inventory. The shotgun isn't the most powerful weapon in the game, but when your character first gets it in FO1, it makes a world of difference over the starting pistol or the MP9. Soooo...
Fallout -- tie between the sniper rifle and turbo plasma
Fallout 2 -- speed-load magnum (w/fast fire perq) and combat shotgun are sentimental faves, the EC assault rifle is mighty fun for cheap bursting w/o reloads, tho
Fallout:Tactics -- the numerous combat shotguns are cool, but the Browning M2 is king of the midgame before the robots show up, and the energy/emp weapons take over. Even then, the M2 holds its own if you can get enough 50 cal. Grenades are very cool in FO:T too, for breaking up those "standoff" situations with some indirect fire.
Biggest disappointment: Flamethrowers. They yield the best death scenes, but such very crappy range and heavy ammo. I'd like to see just ONE good WW2-style bunker-busting flamethrower show up in a Fallout game sometime. Still, once you get some power armor, a rampage through the New Reno casinos with a flame thrower can be extremely entertaining.