I mostly like to play Tracer due to her insane mobility.
Other choices depend on the team composition.
I don't have a favorite in the support types, but Symmetra is my least favorite of all heroes.
I generally don't pick Tank because that role mostly gets filled by others, but i like Reinhardt and Roadhog the most from that type.
For defense i tend to always pick Torbjorn because sentry.
For assault i tend to always pick Tracer, though Genji is a very nice one too.
Pretty cool stuff.