The ones I use the most are the memory counters. All of them. I know that they aren't specific to ASP.NET, but the only problems I've ever had with a web app were memory issues.
Excessive heap, gen 2 collections and % time in GC are the most important ones. If your time in GC is spiraling out of control it's a sign that your UI and viewstate are too big. A large heap and lots of gen 2 collections says you're keeping too much stuff in memory (inproc session state, for example).
Regular ASP.NET apps based on web controls require lots of objects being created and then destroyed quickly, as a page is reconstructed and then disposed. High gen0 collections isn't bad. Its when you start seeing lots of objects make it into gen1 and then gen2 that suggests you're either leaking memory or are holding onto too much state.
