I don't know if I have a "dream" locomotive, BUT.... I can say that after getting back into the hobby, I'm more of a modern diesel fan. Never really cared for steam, however, as of late, my feelings have changed. After being at DOC's house and then Patrick's, my goal is to get a steamer. DOC has a couple of Cab Forwards which I've become extremely fond of, Scrapiron Scher was selling one, but I couldn't afford it. If I had to pick a "dream" loco though, it'd be a toss up.
Cab Forward
Challenger
Big Boy
Southern or Pennsylvania NS Heritage
Just bought, but haven't picked up yet, an Imperial Railking Norfolk & Western Allegheny from a forum member.