Tire dynamics are far too complex to reduce to one simple number. The amount of grip a tire provides is dependent on too many different factors.
Also, IMHO sheer grip is less important than how the tire feels as that grip begins to go away. A tire that instantly loses grip with no warning- or turns vague and mushy- can prevent you from effectively using all the grip that's available. [
As for the decision-making process, my favorite resource is Grassroots Motorsports magazine's periodic tire tests. They track-test different tires on the same car(s), publish the times, and publish realistic feedback from test drivers who don't simply parrot the tiremakers' press releases. [rolleyes] Of course, it helps that I have access to every back issue of GRM since 1993. [cool] The last tire test they did was in the August 2008 issue, but I don't have my collection handy, so I don't know when they did the last one before that.
What kind of tires are you looking for, and how will they be used? [confused] These are for actual street use, right? I assume based on the "rains" comment that you aren't
looking for a set of super-sticky competition tires for a racing class that requires a "street-legal" DOT-rated tire.