Jirachi is definitely an interesting option... While it only existed for ADV and DPP, its dominance is pretty hard to deny.
I like the idea of Starmie at #5, Borat. Few Pokemon have stood the test of time as well as it; it's certainly a great offensive presence and among the best (if not the best) offensive spinners ever. There's definitely a reason this thing has been nothing but OU every generation.
Skarmory is also quite metagame defining. It's been the go-to physical tank since GSC, and one of the best Spikes users to be found in ADV and DPP. This niche allows it to function on offense, stall, and balance alike :3