I'm with Tyson. What are you referring to? Locomotives, rolling stock, structures, etc.? Also availabiilty plays a large role. In locomotives I prefer Atlas to the extent they have what I need. Kato is also high quality and I have a few Life Likes that are good. In rolling stock, MTL is the standard, but I have a ton of Intermountain because only they produce the Canadian grain cars. Atlas is also producing some fine rolling stock. Again, who is making what I need?
These "favourite" type of threads always seem a bit simplistic to me . . .
David