How do you measure if you're good with a Hero?

k/d? Win rate? Damage? Heals? Gold medals?

I played a certain number of Heros during the beta, some I enjoyed more than others, but how I am supposed to figure out what I'm best with (of course for a certain situation)?

EDIT : vid here where he says that 1000 damage a minute is good (https://www.youtube.com/watch?v=Wmgep81i0Us). I around the 500 mark for my top damage dealers.