While I haven't been maining one hero, I'm playing tanks/healers/supports in about 60-70% of my games. I'm finding that I enjoy the play-styles and more objective-oriented roles over Assault characters.
The heroes I seem to be putting a lot of time into, or doing best with, are Lucio, Zarya, and Winston. They're all generally very strong and I'm really enjoying the nuances and play-style of each one.
Discord