best character to main?

Honestly, it depends on your style of gameplay..

If you're really aggressive, Jett,Rayze or Reyna should be fine (easier to start with).
If you're more of a defensive player, consider taking Brimestone, Cypher or Sova

I'd think that Omen and Killjoy could handle both roles, but they're hard to play with.

[Vyper is obviously one of the worst characters]