Should you one-trick or flex?

This is a tough question that all new players find themselves asking, and it really depends on what you enjoy more, but assumming you only care about gaining SR it's better to one-trick.

It's better to be a master of one hero than a platnium of all heroes. And if you devote all of your time to a single hero, you will master them eventually. Who would you rather have on your team, a GM Genji or a plat flex?

Now being a one-trick doesn't mean you should play your main every single game, sometimes your main is locked by another player or you've got no healers or no tanks. In these situations you're going to have to pick another hero, your game-sense and mechanical skills will cross over but don't expect to be comfortable with another hero's specific skills. Mercy is a great pick if you need to play support as she requires no hero specific skills. Hog is a great pick as well for the same reason(though not to the same extent).

In conclusion it's better to one-trick for SR as long as you switch to heroes that skills transfer well to when necessary. Five heroes I'd recommend one-tricking would be Genji, Tracer, Ana, Roadhog and Widowmaker.