i think that blood angels could be a serious contender for both.
in 750 you could field.
honour guard with sanguinary priest, chapter banner, 2 meltas, powerfist, jump packs
3 death company with jump packs
2 5 man assault squads with powerfists in rhinos
between dante, the honour guard, and the death company everything dies very quickly. the opponent should have 1 or 2 turns to fire at them before everything falls apart. supercharge the rhino engines and hide the jump packs behind them with the rhinos smoking first turn. as long as the rhinos don't both immobilize(happened to me first turn last game at a tournament after my opponent stole initiative) then you have your actual good squads jumping then running while shielded from view. at 750 the enemy might be able to destroy both rhinos, but probably not both troop squads. the only thing that could beat this list is high initiative assault armies, or armies fast enough to stay completely out of reach.
either way its simple to play, and hard to lose with. if dante and honour guard and death company all hit assault turn two. against marines they would cause 2 melta deaths, 2 bolt pistol wounds, then 5 power weapon wounds and 7 regular wounds at i5, 8 regular wounds at i4, 3 fist wounds at i1.
if they can catch more than one squad into combat they might not massacre everything, and might just kill both by the opponent's turn.
i think that one of the main problems would be a slaanesh daemonettes list, or lash/vindicator