Definitely Republic Commando 2, for two very good reasons.
First, it would probably have sweet coop, the first game was practically made for cooperative play but they clearly didn't feel it was worth the resources. Today, with the obvious popularity of games life Left 4 Dead it is very likely Republic Commando 2 would have a full coop campaign.
Second, and most importantly... It would be goddamn Imperial Commando. You would be taking orders from the Sith and attempting to exterminate Jedi. The prequels may have failed utterly in making this seem cool, but I am almost positive a game in which you play a squad of clones post Order 66 would be epic.