It'd be 4 for me, personally. I just dig the idea of a literally person-powered death robot xD 

Plus it just has a bunch of cool in-world things going on between Extremis and the mysterious scientist who got hired on. Hell, if you wanna add another layer of intrigue, this could even be a project by Justin Hammer; seems up his alley, given the human testing and poorly tested war machine lmao, and it fits even moreso given it was originally designed to become a contestant to the Iron Man armor, and there are few people on Planet Earth who hate Stark more than Hammer.