Switchers, the NW/SW is larger then I like. Docksides/Vulcans/Plymouths for 3-5 if they aren't too heavy. Adding to loco weight to guard against wheel slip might be a good idea. I've never needed to lighten a loco
Road/switching combination duty, diesel, the RS1, but I don't have a clue how well they do on the turnouts.
My bias is the "2037". Go figure. But, if you can find any version of this 6 driver unit with magnetraction, Prairie, St. Paul Pacific, Adriatic, etc. you've got a winner. I'm not 100% sure but I think the 2037 has the nicest rods, rails, etc but I think another has nicer, bright rimmed drivers? (I do like the alum stacked Ks a whole lot too).
I can pull 19 of my heaviest. With 18 or less on it, against any other, its still the fastest thing I own, and least likely to roll in a bend at warp speed. After adding a couple diodes(free ones from a coffee pot) it slowed its top end, made it smoke even better/sooner, and creeps out of bed in the morning more nicely too.
With just a touch more size (& huge cab kickout) The C&O Berk jr is the absolute smoothest thing in MY* turnouts.(*each turnout can be unique! No way out but work on one of the two, engine, or turnout)
Creative cutting of the corner on the cover of the 0-27 switching coils can be used. Trimming off corner allows many string-line overhangs to fit without hitting. JB Weld the open corner shut, and shape/carve/file/sand to taste. It actually blends into "lionel black" excellently. I have a plastic scout tender you can't easily tell the repair till I show you its a whole front corner, from right of center looking at it, around the corner to near the rivet line, top to bottom! (Every internal screw boss too)(wood/metal homemade frame ). Anyhow I can run a tmcc e-44 rectifier without the dreaded "box bump" on those.
Some semi-scale GG-1s will work, some need mods/mods on lead cars, others no hope.
One thing to keep in mind, on the NWs for sure, some have strictly O gauge rollers(longer arms) and they can fall off O-27s center rail in the tighter bends. Most 0-27 is a 4 digit number vs 3. Many can be replaced with the 0-27 version. Anything could be bashed/worked out roller wise for different handling characteristics in turnouts if you need to. A single tether from a tender/car roller to the loco does wonders.