A tough question if ever there was one.
I like Monogram stuff for the workability, they're easy to convert and correct if need be and the price is usually right for the quality.
I like things like Pavla for the sheer esoteric nature of the subjects, yeah they ain't for beginers but where else are you going to find the rare stuff if not for the limited run guys?
There are some companies that I have a love hate relationship with. I love ICM for their line of 1/48 Spitfires, I still haven't quite forgiven them for their 1/72 MiG-31 Foxhound (Is that kit some sort of holdover from the KGB's "Book of Better Cold War Torture Techniques"?!) Oh, the humanity!
Even the best manufacturer can unleash a dud on us all. I try not to play favorite because of that.
A final note: when I want to try a hypothetical subject, you can't beat the old Airfix and Heller 1/72 kits. They're so basic and spartan that you can do anything with them and if you screw up, they're usually cheap enough that guilt isn't an issue for you when the bank statement comes in.