Well if we're talking T2K weapons, you just can't go past the PzF-11-1 from the 1st ed heavy weapons book. Light weight, high damage, good penetration, and a respectable blast radius too.
Although depicted on the 2nd ed heavy weapons cover, there's no stats inside the book for it.
As for what I've personally used, that's the M72A6 LAW (aka "66" due to the size of the projectile) and 84mm Carl Gustav. Both are a little outdated, but still effective against most likely targets.