They can be dual-typed, but at least one of those has to match the type you choose. What type are you choosing, and what’s your team look like?
I would have to choose Bug-type, of course. Team composition would be Galvantula, Yanmega, Heracross, Centiskorch, Genesect, and Rabsca. As to why… I just like 'em, lol. I’m not much of a competitive player. What would your team/type be?
Dark type. 100% just because I play by the rule of cool and my favourite Pokémon belong to that type. Not sure about the entire team, but Houndoom, Mightyena, and Umbreon would be there for sure.