Which model kit manufacturer is the best?
Another tough question to answer, because all the manufacturers have their pros and cons.
Right now, Revell-Monogram and Tamiya are at the forefront of the plastic car kit market (and they were the only two companies to release all-new kits in the U.S. in 2001). For some insight into the pros and cons of those two manufacturers in particular, please read:
Tamiya vs. Revell - Let's get it on!!
Sometimes, though, it doesn't matter because only one manufacturer makes a kit of the car you want, so you don't really get a choice!
In the end, it's the subject that matters, and any extra work you put into it will reward you ten-fold!