5th ed?
My "205" Demon Prince. Mark of Tzeentch, Wind of Chaos, Warptime, Wings.
18" threat radius, powerweapon-flamer that re-rolls to-wound, re-rolls all hits and wounds at WS7 S6. Ignores FnP and 2+ saves, strikes at I5... he eats whole squads at a time.
4th?
Demon Prince with Stature, Speed, Strength, Dreadaxe, Mutation, Furious Charge. Usually hit at I6,WS 6, S7, T5, 5 attacks that allowed no saves of any kind; had fleet and a 12 inch charge. Every now and then i'd give him demonic aura, because he quickly had every heavy weapon in the game pointed at him. I really miss him.
I'm big on nasty CC demon princes.