What is your pick ?
What scale? With or without DCC? With or without sound? Too broad a question to have an answer. In G scale you can even get live steam if you have several grand laying around in your bank vault collecting dust.
The Nickel Plate Berkshire - hands down.
Can’t give you an argument against that pick
The real ones
Boston and Maine R1d
NYC Niagara
My favorite is the Rio Grande 1700 series M-64 4-8-4’s built by Baldwin in the late 'twenties. One of the prettiest 4-8-4’s I’ve ever seen.
Here’s a model of it produced some years ago by Key Imports:

Tom
As a diehard, lifelong ATSF & IC fan, I have to say the best looking steam loco is the NYC Hudson!
For the ATSF, their large Northerns (37xx) are their best.
For the IC, much as I hate to say it, those Paducah style box sand domes just kill the aesthics of the locos. And to make matters worse, you can’t even get a decent HO model of them!
Hey, the above is just my opinion, and I won’t argue one way or the other with anyone that disagrees.
ENJOY !
Mobilman44
Japan National Railways C62 class 4-6-4.
Chuck (Modeling Central Japan in September, 1964)
Mobilman
I agree the NYC Hudsons were very nice looking locomotives, but I was raised watching the 2500 and 2600 series Illinois Central mountains and the sand boxes looked just fine to me. I was surprised to find other railroads had rounded sandboxes.
The trick is to pace them at 75mph to get action shots or up close.
CZ


My all time pick might be the C&O Greenbriar. A very nice locomotive.
And it might be?? This is one of those questions that has about 100 anwsers!!
I personally like the GN S2 a lot.
If BLI actually ever made it, I’d be tempted to get one and use it for excursion service or something
I’m suprised there hasn’t been a nod to the AA 20-01 (4-14-4) or UP 9000 (4-12-2) locos
LNER A3 Pacific I love British trains http://www.youtube.com/watch?v=ygF5_98PTrQ watch the first couple of minutes in HD, it is the new Tornado, wonderful stuff
Reading T1!
Nick
I vacillate a lot the Nickle Plate Berk–the USRA heavy Mountain and the Lackawana Pocono
I can’t decide betwen the C&O T-1/PennsyJ1 and the UP 9000 series long-boilered three-cylinder beasts with the flying pumps. Eventually I’ll have one of each. [swg]. At least, that was the plan…BLI seems to have gone soft on their offer of the 4-12-2.
-Crandell
Bingo! For me, a classy and timeless design. I will say that the NKP Berkshires are not too far behind though.
Tom
For me I think the best looking example is CNR’s class U-2-e Mountain. They are a very clean looking loco especially with a Vanderbilt tender.
Lawrence Stuckey, a fairly well known photographer of the CPR thought so too.
CN Charlie
Gotta love those chunky PRR steamers
How about an E6 Atlantic?