Dodrio or Butterfree.
I always use a Dodrio in personal runs of FireRed/LeafGreen, I actually started using it because Tri-Attack was always one of my favourite moves, even though i didn't think it was very powerful. Then I learned that 80 base power +110 base attack +STAB = God
I actually only ever used Butterfree because my first ever shiny was a Caterpie in FireRed, and I thought "Don't care if it sucks, I'm using this thing." I learned that Butterfree is actually pretty useful and now I love using it in personal runs.