That is a tough choice from that list, a lot of my favourite mercs there. Stoker and Nader are both really fun, great second dps mercs. Stoker is just slightly more usefull, although nader has nades and martydom, she is only for kills, whereas stoker does a great job of killing and supplying his team with Ammo. Aimee is good if your a really good sniper. Phoenix is my favourite merc, I have countless hours with him, he heals himself, allies, and revives himself and allies, which makes him good for just about any situation, you can stick with the team or go wrecking on your own, he also gives loadddsss of XP and he is fast.
So, If I were you I would go with Phoenix, if not, then stoker.
