Normal/ Flying. There are 24 counting NFEs, 11 counting only fully evolved Pokemon (Swablu changes type).
People only say Fire/ Fighting is overpopulated because the last three Gen's fire starters have ended up as Fire/ Fighting.
Pokemon DB's Dual type chart
Grass Poison at 14 including NFEs, 5 only fully evolved
Flying Bug at 12 including NFEs, 7 only fully evolved (excluding Scyther since it changes type)