So far, my favorite has been the SSI Stress & Rescue class (about the same as the PADI Rescue Diver class). It teaches how to recognise and deal with stress in yourself and others, which is something you can use all the time. It also teaches how to handle various Bad Situations, including a panicing buddy. You (hopefully) won't need to use this very often, but it's good to know.
Next in order of how valuable they were (to me) is Nitrox, Deep and Wreck.
It's interesting to note that the drysuit class covers the technical aspects of using a drysuit, and describes how to not kill yourself in one, but it took me over a year to get as comfortable in my drysuit as I was in my wetsuit.
Things like the "Deep" class are helpful in that you get to experience diving deep, learn what to expect, and how to handle it, learn about additional risks, technique, equip, etc. while accompanied by an instructior, however as with the drysuit, it's really a practice-and-comfort-level thing, and you still need to do more dives on your own to get comfortable with it.
Terry
CUunderH2O:
<QUOTE>Pay attention to what dives you are offered as part of the AOW. Some of them are less useful than others.
</QUOTE>
Which dives are more useful? I'm thinking the night, deep, drift and u/w navigator ones would be good. Drysuit, too, since a lot of people dive dry around here.