I voted SH3, but I just got SH4 and have played around with it for only a couple of days. First impression was meh but it is starting to grow on me a bit. I've already started adding some mods and am wrapping up some tutorials before I hit the high seas to wreak havoc.
I think I like the Atlantic though because of the underdog mentality that I get later on in the war when just getting home alive is a huge accomplishment.
I'll reserve the right to change my vote in a couple of months after playing SH4 though, just in case.