Although the only Flyer that I own is one plastic Hooker tanker (might be converted to On2½ one day), I would have to say that my favorite would be the Gilbert EP-5.
Speaking of Flyer smoke, I could never stand the stench of it, Lionel was much less odiferous! At the 2014 Tallahassee train show I took this photo of a flyer steamer with its smoke unit cutaway:
I also shot a short video of the engine's smoke unit operating: