Bachmann Spectrum...nothing else is close.
I certainly love what BLI/PCM has done: C&O 2-10-4s, decent C-16s (the driver spacing is way off), and many beautiful prototypes, but they are not the value that Spectrum is.
Bachmann Spectrum is my favorite because they give me a locomotive that looks just fine sitting next to BLI, P2K, and such...but its costs are in line with a value brand such as IHC.
IHC is an odd manufacturer. Their 2-6-0 is wonderful, but their 2-8-2s, 4-6-2s, and 4-6-4s, are in my opinion, are just plain poor. I also enjoy their OO scale (which is sold as HO) 4-4-0. Love that model.
Mantua was fine, aside from their almost complete lack of details. My Mantua Mallets could out pull any other plastic locomotives prior to the Spectrum 2-8-0 launching the revolution that gave us BLI, Athearn Genesis, P2K, and such.
Rivarossi...that's also an odd one. Their 1990s models were
nice locomotives. They finally brought their internals up to the quality of their details. Still, their flanges were a drawback...I didn't like their gray drivers...and I didn't like it that my 2-8-4 had 65" drivers....65" drivers, WTH? I'm sure it was related to their old pizza cutter flanges.
I thought Athearn Genesis was ok...not as good of pullers as Spectrum...details were slightly less...and they were more expensive...I'm speaking of the 2-8-2 and 4-6-2.
For British and Thomas....I think we can break them into two catagories....
Rivarossi is owned by Hornby...so Hornby British/Thomas is Rivarossi.

Bachmann USA is produced in the same factory as Bachmann Thomas and Bachmann branchline (which is essentially equivalent to Spectrum)...so Bachmann Spectrum counts there.
Roundhouse kits certainly were fun...I have a hard time seeing how RTR Roundhouse locomotives could be as enjoyable as a kit I've built (except maybe the shay

). I too hope Athearn gets its house in order on this. At least we still have Bowser for kits.