Another vote for PPB. I had done a private buoyancy session with an instructor after getting OW cert and buying my equipment - at about 20 dives. I decided to do it again as part of my AOW, as by that time I was settling into some habits and wanted to recalibrate my technique, buoyancy and trim. I did it as my first AOW dive to set myself up properly for the remaining dives. I did wreck and boat as deep dives, so was able to reinforce the deep instruction as well. Also, by doing PPB first, we worked on that for all the dives. So, for example, when doing the (deep) wreck dive, I had to use good buoyancy and trim to move through the wreck, and my instructor kept observing my buoyancy and trim and commenting after each dive. By the last dive I was in pretty good shape. Nav was kind of a waste (I know how to use a compass), but is required - so I got checked out for about 15 minutes, then we finished as a normal dive, continuing to work on buoyancy. In reality, regardless of what courses you choose, at least for me, it was more of a general "let's turn you into a better diver over the next 5 dives" session. Deep, nav, wreck, etc. were just different scenarios in which to get one-on-one instruction to make me a better diver. And I think it worked.