The standard answer to the question "What's your favorite locomotive" is usually "Whichever one I'm running at the moment," and there's some truth in that. But in my postwar collection, the hands-down answer is my Korean-War-era 2026 -- because it's the very first locomotive I ever owned, brought by Santa at Christmas, 1951. Still looks and runs great.
Second postwar favorite is harder to say. My 2032 Erie A-A diesels? My 736? The 2046? 681? 675? Much harder to decide.
In my scale lineup, it's my Lionel 783 Hudson. I never thought I'd be saying that, because when I bought it NIB, it would barely run at all, and I started to think I'd be relegating it to display-only. But after sinking months of patient work into improving its performance, adding 700E valve gear and, of course, weeks of break-in time (no, I'm not exaggerating), it runs outstandingly well and is always an attention-getter on my layout.
Second-place favorite scale locomotive is probably my Lionel 6-18309 Reading Fairbanks-Morse that pulls my RDG 17674 caboose at the end of its trains. No, the paint scheme isn't colorful like most of the other Lionel FMs, but it's powerful, runs very well and has an all-business working-locomotive look that I like very much.
And, since everyone seems to be talking about them, I'd have to add my 623 and 6220 NW-2 switchers. Smooth runners and nicely detailed. And I disconnected the bell in the 6220.
There are too many scale honorable mentions to list here.