Start playing them in the practice range or qp until you find one you like. Nobody is gonna be able to pick a character YOU enjoy for you. But if you want my opinion I think Soldier is a good jumping off point.
Well, Soldier is an all round dps. He is a hitscan so he is useful almost all the time. Now, Reaper can be useful in many occassions but that depends more on the enemy's comp,plus Reap is easy to learn. I'd say start with Soldier!
Soldier got me to diamond. Mcree got me to master's. Soldier is so easy and has little punishment to missing shots/abilities. Once you're great with him, you will do major damage.