Noawadays, I use Gretchin as scoring units, as cheapo tarpit units or (more situational) ablative wound markers for my Nobs. Yes really. I do hate Assault Terminators so much, so assuming I can kill their ride, I try to take advantage of Ork charging radius, to use Grots to pull attacks away from the Nobs; after all, a Thunder Hammer is *less* effective against Grots.
That said, the best Grot units are Killa Kanz; they're so adorable how they waddle into battle, happily blasting away the foe. If we're talking Forgeworld stuff, then Grot Tanks, Grot Sponsons, and Grot Bombs also deserve mention.