Some of my own preferences:
My old standby is the SD80/9043MAC, which I believe to be a popular engine. Among modern power, it's definitely the best. I don't like the more angular nose on the SD90MAC-H II. Interestingly, both GE's and EMD's current products produce the same reaction: nice body, butterface.
Speaking of widenoses, I prefer them without a window in the nose door. And I like the 3-window cab.
Like most people, I also like the GP60M and standard-nose B40-8, among "modern" power. A surprise was finding I liked the standard-nose C40-9.
Going into second-generation power, I find I like the 20-645 EMDs: SD45 but more so the others: F45, FP45, SDP45 (I like the SD80MAC, how can I not like this?), SD45-2. And I suppose the more esoteric "flares": DDA40X, GP40P-2, GP40X, SD45X...
Just as much as those, the big Alcos: C630, C636, M630, M636, M640. Also C430.
Among switchers, the Flexicoil-truck SW1200/1500.
First generation: Yes, there are locomotives better-looking than Alco PAs. FM C-Liners and Erie-Builts. Even among Alcos, I find the FAs more proportionate... favorite is the FPA-4.
FP7/9s look too long, preferred Fs are the first two phases of the F3 as well as the FT. Among Es, however, I prefer the E8/9 over the earlier models except the E2. I guess I have a weakness for portholes.
Best-looking first-gen hoods are Alcos: RS-2/3, RS-11, RSD-7/15. But also the SD7/9 (actually like the SD24 more - good call, tj48).
First-gen EMDs look bad with chopnoses... but worse with original (sloped) low noses.
Favorite first-gen switcher: S-2 (specifically prefer it over the smaller-radiator S-1 and S-3, and prefer Blunt trucks). Or maybe Alco HH-series.
The DL-109 is NOT ugly.
Steam:
NYC S1 Niagara, L3 and L4 Mohawks, J3 Hudson, A2 Berkshire (yes, all of them). Canadian Pacific G3 Pacific (later version), P2 Mikado. SP unstreamlined GSs, AC-9 Yellowstone. NP Z-8 Challenger. South African 25 condenser. East German 01.5 and 18 201. SNCF 232U1. Gresley streamliners. Bulleid Pacifics (yes, seriously). Soviet P36.
Also some tank engines: Boston & Albany 4-6-6T, Great Western pannier tanks, German 86 and 95/Prussian T20, East German 83.10 and narrow gauge 2-10-2Ts, Japanese E10 2-10-4T. Note that most of these aren't 0-x-0s.
The countries with the most consistently attractive steam are Germany and Japan.
Features I like:
Skyline casings, disk drivers (particularly Scullins), Elescos, all-weather cabs, Vanderbilt and centipede tenders. Solid pilots (though I've discovered I don't notice the total lack of pilots on British and European steam).
Dislikes:
Russian Decapods and other such steam with small high boilers. Most 4-x-0s. Horizontally-slatted pilots. Inside-bearings on four-wheel trailing trucks (these hurt some otherwise-perfect European engines, the Norwegian "Dovre Giant" and the DB rebuilt 05). Cabbage stacks. Bell-like or flared domes, and overly high domes in general. Wooten fireboxes, though camelbacks make them acceptable. To a lesser extent, Belpaires. Too-small tenders.
Fan of late and early Conrail... also transition-era PRR, 70s Santa Fe, BN and SP, 70s-80s eastern CN, pre-merger-era UP, heavy electric operations in general, dieselized narrow gauge, transition-era DB and DR, modern EFVM and Brazilian railroads in general... why bother trying to list them all?