Tough one.
Are we assuming Superman instantly has the same presence in Marvel that he does in DC? For example, Superman in DC is renowned throughout space/time, and stands as the moral compass of DC Earth. In Marvel, Steve Rogers obviously comes closest to replicating that character persona, which IMO would lessen one of Superman's most valuable attributes.
Depends on whether or not Superman's history comes with him and what you mean by "most valuable". Traditionally, the Big Three have always been the most valuable assets of the team: Steve as the leader/tactician/moral compass (pretty much every Avenger said that the team would have eventually collapsed if hey hadn't found him in the ice), Tony as the brains/financial backer/spokesperson, and Thor as the powerhouse.
So really, it depends on when Superman joins the Avengers and whether or not his history comes with him or if we're to assume he has a completely revamped Marvel history.
Originally posted by Branlor Swift
Isn't Wolverine on the Avengers right now?How could Superman ever be MVP with Wolverine, Thor, Cap, and Wonder Man on the team? Don't even get me started on the other team.
Superman isn't good enough for the big leagues in Marvel, he would be one one of the unheard of teams from the 50 state initiative.