What HO vendor produces the best quality UP Bigboys and Cab Forwards?
Remember, the Cab-Forward is NOT a UP engine by a long shot. Of course you knew that, I was just being 'picky,' LOL!
My BLI AC-5 cab-forward (one of the original runs) is an absolutely terrific locomotive--smooth running and EXTREMELY powerful. It's actually the only dual-mode 'sound' locomotive I'm keeping (I'm DC), because I was able to program its starting voltage down to about 4-1/2 volts. It can almost (and I say "almost", with caution) be run in tandem with my straight DC locos.
MTH is coming out with an AC-6, which is the true 'transitional' 4-8-8-2, between the AC-4/5 and the later 'streamline cab' AC 7, 8, 10, 11 and 12. It was the one with increased boiler pressure and tractive effort, and the first to have the air pumps transferred to the boiler 'front'. They're releasing it in both 'flat' cab and 'streamline' cab (several AC-6's had streamline cabs installed after they arrived from Baldwin). I don't know how it will run, but the production photos look impressive. It's the usual MTH 'triple' mode, DC, DCC and their own DCS, but if they somehow manage to put out a straight DC version, I'll be first in line.
I've heard varying reports about the Intermountain second run of their AC-12's, so I can't really say much about them. They're certainly handsome, but I know the first run had a lot of problems with speed, pulling power and tracking. Hopefully, the second run has cured that. We'll have to wait and see.
I know that BLI is planning a 'hybrid' version of the AC-12 (which would work for the other series of Cab-Forwards from AC-7 on up, with a few detail changes), and it will be interesting to see how they turn out. If they're anything like the first run of the AC 4-5's, BLI should have an absolute winner.
Sorry, can't help you on the Big Boy--but I know that a lot of other posters will have a lot of information.
But just for kicks, here's my all-time favorite cab-forward, it's an old (1964) Akane brass model of an AC-8. Runs like a Swiss watch, hauls like a team of oxen.