In order that I did them...
* Open water (entry level stuff)
* Advanced open water (40m deep dive, navigation, night, currents)
* Nitrox
* Wreck
* Solo diving (I don't usually solo dive, but it does make you a safer diver)
* Rescue (main lesson was don't put yourself in a position to get rescued)
* Posiden Recreational Rebreather (I had a cheap opportunity on holidays and I was curious)
* Advanced Nitrox (limited decompression diving to 45m)
* Technical diving introduction
* Normoxic Trimix (extended decompression diving to 60m)
I average about 100 dives a year, about half on holidays (in the tropics) and half in Melbourne (cold water). Most are below 30m with a handful beyond 45m.
I've got no desire to teach at an entry level unless it's a friend. My wife has AOW & Nitrox, she's done it all with professionals but I regularly put both of us through various drills. I am thinking of doing a Tech DM course and I'll probably go all the way for technical diving eventually.
If you're looking for a course after AOW, do Rescue. It's mostly surface skills, but quite challenging and will make you think about how you go about your diving. The last thing you want in diving is to need to be rescued.
Other than your obvious equipment, probably the most useful thing is a surface marker. Aside from marking your position, it's an easy way maintain your depth on safety stops. It also doubles as a useful surface float while you're waiting for a pick up. Most people don't carry one or the one they have is ineffectual, and a lot of people don't know how to use them effectively underwater.