Brubaker's Cap is, IMO, definitive. All of the Ultimates are pretty much pod people who coincidentally have the same names as the Avengers (Ultimate Iron Man in particular is an entirely different character with a completely different backstory).

I've seen fans suggest that the mention of the French Resistance Brubaker stuck in his Captain America was a deliberate response to Millar's France joke in Ultimates, though that's just a theory.