Brandon,
It is a very trick question to ask who are the leaders, because you will get as many answers as there are manufacturers. If you use the search here on CC concerning mfr comparisons or "which kit?" you will start to see the different flavors available in the ice cream shop...
I believe that the "best" manufacturer is the one that has the product that suits your desires the best. All have strong and weak points.
Use the info you find here in the threads. Visit
www.cobracountry.com and
www.cobratrader.com (and others) to see other opinions. Narrow the list of manufacturers down to 6 or so and give them a call and/or a visit. You will pick up a lot of "knowledge" about the manufacturer by just visiting their shop. Shell Valley and FFR have sold a lot of cars, but they are by no means the only players you should look at.
You can also search here for users that live in your area. Talk to them and see what they've done. Heck, give me a call and I tell you all I can about Midstates and why I picked them...