My primary interest is still the 'transition' era but I appreciate some of the modern large diesels too - the Santa Fe AC6000 is a favorite. Personally don't particularly care for the SD70's for some reason, purely aesthetic however. I like all PAs, F units, E series, and some of the variants as well as the DL109/110 and the RF16 Sharks. Some early road diesels and switchers too.
Steam overall is my favorite although I only have a few at present - NYC Hudson, MR S3 #261, Allegheny 2-6-6-6, N&W 611, and two early Lionel Generals plus a MTH 10-Wheeler so my diesels currently outnumber my steam.
Just received a beautiful Green JLC 5 stripe GG1 to pull my Williams 5 stripe 'Blackjack' dummy as a double header which is my sole electric locomotive.
Love 'em all!