Mister Negative is the core of the deck, making all of our cards hyper efficient. Psylocke lets you play him a turn early, and Crystal pulls the affected cards out of our deck after.
Jubilee and Lockjaw help with finding him on time. Crystal does this too, in a pinch, but means you'll only get one turn with Negative.
Sunspot soaks up extra energy on turns before we have Negative, and can also draw cards with Adam Warlock without too much effort.