Team leader would be spawn, he has worked in teams before and has spec ops training combined with all of the demonic powers at his whim, and with his suit being alive and wanting to keep spawn alive it will help him in combat and keep him alive with it's healing powers, only down side to that is that he has a limited power supply at his disposal.
Deadpool would be 1st pick, hard to kill, skillful with weapons and an awesome sense of humor that he could use to confuse the enemy! Also not dying from mortal wounds is a plus. Hopfully he doesnt get the rest of the squad killed.
Batman would be number two, he's rich, has smarts and has good hand to hand training not to mention he can disorient his foes with any number of tools available, being able to create devices that will help fight his foes is also a plus.
Last but not least id have the green lantern with his super ring of gloryness that can make anything imagianble, at a whim, his ring of power also warns him of any imminant threat to his self so incase the aliens are shape shifters he'll have some warning.