S.T.A.L.K.E.R's RPG-7, Gauss Rifle and all rifle mounted grenade launchers.
In of themselves they are fantastic, long range destructive power that amounts to a one hit kill on practically every enemy, even the Pseudogiant.
Problem is, they weigh so much you can carry a few hundred rounds of ammunition instead of one of these, which is far, far more useful in the context of exploring the Zone and surviving the X-labs.
Plus the ammunition is rarer than hens teeth until the very end of the game, when you don't need them anyway. So far I've found about 15 GP-25 rounds (About 7 fired at big mutants from my Fast AK, the rest waiting for Pripyat and a Groza), 2 RPG 7's (and launchers) and a lone, single round for my M203, which I'm holding onto for an emergency.
It sucks to have all these explosive firing weapons and not being able to use them because they're too heavy to carry around and the ammo's too rare to fire at anything but the rarest enemies.