As someone who was crazy enough to buy every Pro in the invitational, I can say that my favorite pro is Captain Spark and least favorite is the Gunslinger.
Captain Spark is hard to learn and harder to master, and I guess that's why I love using him. It's just so rewarding getting a grapple kill with him!
The Gunslinger is just plain awful, I play two games with her and hated it. Even with level 4 damage and a few clean shot I never was able to get a kill with her. The only kills I got were from spamming that bullet spam move.