As far as heroes go, I love Nightcrawler because he can both teleport and wall-crawl. Dr. Strange is also cool because he can fly and teleport. My five year old son likes any of the heroes that double jump, especially Iron Man MK1 and Angel because their jumps have a little extra flair to them.
As far as cards go, I Must Break You is the most hilarious card art of them all. Seeing Colossus holding that tiny hammer is fabulous!
As far as tough bosses go, I think Mystique takes the cake. I have yet to finish that mission on Adamantium! She's brutal!