Well, generally, the best stuff is what the in-crowd calls "brass" (you may know that already). It's sorta expensive, but if you want an SP engine, that's where you go.
EXCEPT, Athearn makes a really nice SP 4-8-2. I hear. No one's made an SP 4-6-2 or 2-10-2 in (shudder) plastic. The SP 4-8-4's are everywhere, I think.
But, you asked about the preferred manufacturer. As opposed to importer. I think Boo-Rim would be it. Others, please correct me if I'm wrong.
And, if any manufacturer is listening, I think a LIGHT SP 4-6-2 would b e nice. Because it would also work well as a UP light 4-6-2--such beautiful engines!
The ''in crowd'' (I own one brass loco) just looked at a brass steam site, here goes : SP 4-8-2 :$295 & $275- - - 4-6-2: $395 & $295- - - -4-8-4: $295 & $350- - - - 2-10-4 : $295 & $250. Seems not bad prices, is this because no one wants SP? prices seem reasonable to me. (there were 2 pages of locos)