

All heavy cavalry is very powerful in 6th, but them in particular. Also a shout out to Bretonnia the lance formation was designed for 6th edition combat and due to removal from the front ranks in CC Bret Knights will never get attacked back on a charge. The lords form Vampire Counts are also very strong but that goes without saying.

Fear is incredibly powerful due to the automatic breaks from victorious combats, so naturally Tomb Kings and Vampire counts shine in this edition. Tzeentch is also very powerful since all their units increase your magical capabilities. Khorne warriors, particularly with paired weapons, output more attacks than any other regiment, which means a lot when only the front rank can attack.

Hordes of Chaos can be ludicrously powerful in two variants. I won't try and rank the armies, but I will mention a few armies that tend to be overpowered in 6th edition. This makes all armies fun to play, but it does not take balance into account. This is probably not the answer you are looking for, but one of the things I like about 6th edition is that fluff is taken into account with most of the rules.
