Why heroes are the best for HBM?
From A to T, it doesn't matter quite that much. You can get away with using a lot of different setups, with some heroes better than others. In general though, you want to have the following:
2) Cupid or Val
3) Druid or Ghoulem
You want to fill the 3 remaining slots with DPS. A general rule of thumb is to simply use your best DPS heroes you have at the time.
AA-AE is a bit of a different beast. For these, you have Reflect Demon bosses, so heroes with Scatter are very helpful. Scatter isn't necessary, but it's a whole lot easier. The waves are also a lot stronger, so you don't quite have the flexibility to use whoever you want.
PD, Val, HQ, Molt, Arc, GF, DD, Aries, Ghoulem, PK, HB, and Vlad are popular choices.
As for AF, from my own personal experience, beating it appears to be purely luck based.
I beat it with PD, Val, Gh, DD, Vlad, GF, however, I know some who have had no luck with that setup.