That's a tough one to answer. My father got me into this in the 50's. He liked all steam and diesel. Then he got into Riverossie steam the IHB 080 was his fav. Then he built on this with most all the way to the big boy. My favorite steam was PFM 2662 it was the best.
When he passed I was sent some old Bowser and many tyco steam I built. Then I got into Rocco electrics did a lot of trading there were many really fine loco's. Later on I really got into anything Alco RS-3 , 11's C420 and 424 all atlas and the switchers Sw-7 which were blue box and the baldwins. Then they came out with the SW-1 and HH walters put out I had 4 of each. Then Stewart with the VO-1000. Ended up selling most. Stewart had the Baldwin AS 16 and 616 blue box drive, still have 2. My all time favorite are the Baldwin /EMD and Alco /EMD rebuilds. I did a lot of those and like a fool sold them. It was easy using the proto power GP-18 and Stewart AS-16 or Stewart RS-3 and the old AS-16 frame or RS-3 frame depending. I built a VO-1000 before Stewart came out used 2 blue box Baldwin S-12 shells , removed the headlamp sliced it down the center and ended up with the real hood width nothing bugged me more than the old wide hood look. I still have it. Might have photo's of the others as well. I like EMD F and FP-7 and GP38-2 ,SD-40-2 and GP-35's . Did a few high hoods, still have one gp-38-2 and if it's a low hood I like the short hood version so I cut then down. Did a GP-35 rail power shell and Cannon high hood ends sitting on a blue box modded frame still have it. Had some later Stewart AS-16's Kato drive and F3 high fan Stewart. Love Atlas Alco S-1's ,and S-2. I have one S-4 left like the S-2 trucks better. Made FP7's from blue box F7's and E7 and E8's I had a few proto power now just an old model power. I had a lot of proto power FA's and SD-7's and loved GP-30's used the old Lionel shell then Bachman spectrum came out. Also FM train masters I have at least 6 old blue box and a baby forget the title one Bachman Spectrum came out with have one of those.
The only ones I don't like are the EMD & GE safety cab types. Always wanted a EMD drawing a blank, it's a switcher and road unit MP15 A/C had a concor that was close. And like SW-2's had a kato. Many times I wish I hadn't sold most of them . I still have 2 Atlas FP7's and made one from an Atlas shell hand made frame and have an extra shell one of at least 15 loco's I still need to detail and paint. Made a RS-3 hammer head I still have riding on proto power trucks. I like the rebuilds the best the odd stuff can't say why just do. I'd build another Baldwin/EMD and Alco /EMD if I could find the frames and shells , Front range made some nice GP-7's shells and also GP-30. They had the rivets.
I'm out of breath and at 71 I can still remember.