Really hard to answer definitvely, because the "best" hero is usually a counter to the other team, and of course everyone has thier own opinion. That said, my picks would be;
Best general - Oak
Best Assassin - Beast
Hardest General - Queen of Thorns
Hardest Assassin - .. tough choice, they are all pretty much easy mode, but I guess I'll go with Torch Bearer because he is squishy and it takes time to learn to dual spec ice/fire. A lot of new players go all fire and find out the hard way that you need a lot of HP as a fire mage.