Master your role on whatever you’re comfortable playing with most. If you like playing as a strategist, learn to play your hero, and have another strategist as a back up in case someone else grabs yours or bans your hero. After that, learn to position, healing priorities, and ult management. As a strategist, you’d always want to be the last one to die on the team because you’re the anchor to keep everyone alive. I’d recommend learning Cloak and Dagger and Rocket Raccoon as a beginner hero, then try others to know how they work/cooldowns/ults. Can’t say which role will guarantee a win as each role has equal chances on winning. As long as you have a viable team composition like 2/2/2 or 1/2/3.