Most important counters for Exchange 5.5

Just a quick question.

Please give me your opinions on what are the most important counters to monitor for Exchange 5.5.

These would be critical counters that we would be paged on should they exceed or go below a certain limit.

Actually I would like to limit it to the most important 5 to 10 counters. Don’t wanna over react.

Thanks,

Bisquitlips