Reuniclus, Cofagrigus, Bronzong, Cresselia, and Clefable jump to mind. Suicide Smeargle can usually sleep one mon, cripple another, and set up Trick Room, though he's a tad gimmicky. Dusclops, Slowbro, Porygon2, and Uxie can also pull their weight. I still dunno most of the XY mons, but those are good past gens picks.