Initial poke with Zergling+Marines (no allin, just a small poke which will incidentally win you a lot of games) and later bio+tanks combined with pretty much anything, e.g. roach infestor baneling, depends on your opponents.
Played with some 3v3 partners that single handedly won a game for us by using thors/rines and an army of speedlings/hydras to protect the thors in close combat. Was pretty devastating I thought.
cmon guys you gotta know how to make good compositions. you make unit compositions that counter each other. speedling + hellions gets owned by roaches alone, or marauders alone. zerglings + marines gets owned by one 4 gate roach/hydra + MMM gets owned by banelings alone TZ timing push with marauders and speedlings is a strong start, (think about it, what ever counters marauders get owned by speedlings, what ever counters speedlings gets owned by marauders.) and then tech roach hydra with tanks and vikings or MMM with mutas ling/banelings