I don't know how obscure it'd be considered, but I've taken out the majority of the Elite Four out on multiple ocassions with a Butterfree. As an early game Bug type it has rightfully low base stats, but with decent speed and one of Sleep Powder, Stun Spore or Poisonpowder, it's very easy to get an advantage on your opponent.
The only two obscure Pokemon that I can actually recall using a significant amount in the recent past are Drifblim and Swanna (the latter because it was a shiny).