<p>There are truckloads of counters available in perfmon for ASP.NET.  What are the best (I am thinking of choosing 5-10) that will be the best to monitor in our test environment so that we can feed back to developers.</p>

<p>I am thinking of things like request time, request queue length, active sessions etc.</p>

For a normal (not performance/stress testing) you would be OK with the following:
Request Bytes Out Total
(very important especially for web (not intranet) applications)
Requests Failed
Requests/Sec
Errors During Execution
Errors Unhandled During Execution
Session SQL Server Connections Total
State Server Sessions Active
For the performance testing you would probably want things like:
% CPU Utilization
(make sure you're checking for very low CPU utilisation as well as it might indicate that something is dead)
Requests Queued
Output Cache Hits