What weapons do you think are underused? Not underpowered, like the tomislav, but underused. As in, they're good, but nobody uses them. I think the back scratcher is underused. The added health from healthpacks coupled with the fact that nobody plays medic on normal servers means that it's good outside of competitive.
Stock flamethrower is also underused (or rather, degreaser is overused, but that's another thread.). It has great damage at close range, can airblast, and it can do crit combos. It also lets you easily deal with stickyspamming demomen; just rush them and crouch-jump occasionally. Sometimes their stickies launch you right into their face, letting you burn them to death.