My favorite diesel locomotives are the Class 101 diesel multiple-unit & the Class 142 multiple-unit. I don't have a favorite electric locomotive though. My favorite steam engine is the LNER Class A4.
Steam: GWR Panniers Diesel: Most of the Western Region units, Class 31's and 33's Electric: Southern Region EMU's, Class 304/504 EMU, Class 73, Class 90, the Woodhead Electrics
Well for me it's the Class 47, looks good, sounds good, reliable and nice to drive (most of the time), a proper thinking mans engine. 37s follow close behind for me. To be fair it's easier to say what I hate, Class 14s and Class 33s (and anything made after but including Class 66s as boring modern sh***e)
I used to watch HEA and MGR coal trains hauled by Class 58 locomotives come through Wolverhampton station as a kid, and also pairs of "Railfreight red stripe" Class 20's whenever a 56 or 58 were not available. Around the same time period, I would ride a lot of Class 304 & 310 EMU's all over the West Midlands. Mostly on the Wolverhampton to Coventry and Birmingham New Street to Walsall services. Electrics would be Classes 85, 86 and 87. Add classes 31 and 37 to the above, and you have my favorites list.
Steam: GWR 1366 Class Diesel (Loco): Class 60’s Diesel (MU): 166’s Electric (loco): 92 Electric (MU): 323
from those in-game, I love the Azuma, 395 is also nice cos of the challenge of switching between electricity and signalling systems... from EMUs also 387, wish we had more playground for them... where we could switch between 3rd rail and panto ... I didnt like HST at first, but MML made me really like it and I wish we had some longer route for it
Steam - BR Standard Class 7 (favourite tank engine - GWR 57xx) Diesel - Draw between the BR Class 37, the BR Class 55 & BR Class 68 Electric - BR Class 91 / IC225