The M2HB has the fastest suppression time in the game, 0.74 seconds to suppress a squad at medium range IIRC.
It and the Vickers share a problem in that they do too much damage. Regularly they will focus on a model, that model will die and all the suppression accrued on it counts for nothing. The gun then has to re-aim and, by the time the next model starts eating lead, the squad has killed the gunner or thrown a grenade.