Poison types. There's a huge variety of Poison types out there in terms of appearance/biology (as opposed to, say, Grass types being mainly plants) and as a result get paired up with many other types for some great & effective combinations. Straight Poison types aren't slouches either (only two weaknesses).
Some of my favorite Poison types include Venusaur, Arbok, Weezing, Tentacruel, Muk, and Drapion. It's a real shame GF slowed down on coming up with Poison types. Half they Poison types in existence hail from gen 1. Gen 8 needs to add a few more, I think.