I have raced on Avons, Hoosiers and Dunlops. All are awesome slicks. Best bang for the buck? Generally they will be close in price so I don't think you will find that price is a main driver of your decision. Sizing may be a factor as different brands offer slightly different size options. Also, the type of track driving you will be doing will determine the decision. Will you be on a 4 mile road course with long straights or a tight turny short course. There is no perfect answer as to what race tire to get. You will probably be happy with whatever you get and will need to experiment with your car and the tracks you drive to determine the best fit. Just make sure that you start with the size tire that makes the most sense for your particular car.
You can go to Sasco
http://www.sascosports.com/products/index.php
or Roger Kraus
http://www.rogerkrausracing.com/tires.shtml
They will both have good options and be able to help you select the best tire to your specific needs.
With that said, if you want some better bang for the buck you could go with a DOT race tire like Hoosier R6, Hoosier A6, or the new BF Goodrich G-Force R1. These are all great tires. They will be a better back for the buck because they are cheaper to start with and will also last a bit longer. At around $400 per slick and maybe 3 days of driving out of them can really add up. The DOT slicks will cost about $100-$150 less per tire and last twice as long. They won't be quite as grippy but they are pretty nice. Again, it all depends on the purpose of the tire and your particular situation.