There's a few that come to mind from the vintage sets. I have to say that any ability 'mon that allows for mass energy acceleration once it hits the field is, not broken, but definitely OP. Magnezone, Embour, Blastoise.
I also think the whole concept behind LostGar(lost world, gengar prime) could have been broken if it would have been utalized correctly, and had few more cards that we have nowadays.