Any of these?
Charmander to Charizard, Grovyle, Cyndaquil to Typhlosion, Oshawott to Samurott... or Zekrom.
Or is that cheating? :p If it isn't then mine's fantastically easy. I'll get rid of Zekrom, though, don't reckon too much Zekrom trading goes around outside of the games...
I'm going to go with... Grovyle?