For me, the the nickel drivered postwar 726 Berkshire followed closely by the 675/2025 Prairie and 671/2020 Turbine. They look great, pull well, and like the Timex watches of old "Can take a lickin' and keep on tickin." I have the '47 versions of these marvels of quality American craftsmanship and see no reason they won't be running another 70 plus years from this Christmas.
I like the Lionel's 2025. Great runner.
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Great thing is there is no right answer
Great thing is there is no WRONG answer....
1. Of the relatively new scale-sized Lionel locomotives, I currently fancy my Legacy Reading T-1.
2. Of the vintage traditional steam locomotives, I have to vote for the prewar 226E, which is the daddy of the better postwar Lionel locos. The 226E's in superior condition are fabulously nice runners with a more usable speed range than the PW locos, have a great air whistle, and are more fun to run than (even) a 700E (which would likely be considered the "most desirable").
3. I have a soft spot for any 400E with a 3-pole open frame motor and an air whistle. 
Have fun!
Bob
After careful consideration, I would say my original Lionel 402E Standard gauge loco is my favorite, not steam but the others aren't either. Second would be my 1835 in Standard gauge.
My Opinion: BEST steam loco, Vision Line Big Boy, Don't have one. Too expensive for me.
The best Lionel locomotive is what ever one I’m running on the layout at any particular time. Period 
Favorite engine that I own: Legacy H10. Not perfect, but you can't beat it for the price.
Favorite engine ever: Vision Line Niagara. Smoke, smoke, smoke!
The best steam loco ? I would venture to say that picking the best steam loco from all that Lionel has made, would be like comparing apples to oranges.
The post-war period would have it's best as would modern manufactured locos.
I have to agree with those who said the 675 or the 2025 and other locos with the same type of motors and gears. The 671 and their brethren are also on the top of the list. These locomotives were built to RUN. The Berkshires of the post-war period are also excellent runners and they handle O31 curves well, but not quite as well as the others I mentioned.
I can only speak about one newer Lionel steam loco and that is the LC+ A5 w/Bluetooth I just purchased. For what a switcher is meant to do, it out performs any post-war steam loco that I know of. I have the re-release of the Lionel B6. It is a great runner but cannot compete with the A5 LC+.
