I feel like we've had a thread like this before, but I can't see one at the moment. Moved it into the right forum anyway.
I believe I'd probably have a team of either Grass-types or Bug-types - two of my favorite types, both of which offering a great deal of diversity these days. Curiously enough, the only competitively-viable monotype team I've ever built was a Ghost team, even though I'm not particularly attached to the type. Just felt like an interesting experiment.