Most transformative: GUE Fundamentals. Still the pivot point in my diving.
Useful runners up: My Puget Sound Marine Life ID class (not your usual one day PADI thing, but a distinctive specialty that involved four three-hour class nights and over 1000 slides) and Rescue.
Hardest classes: GUE Rec Triox (which I never succeeded in passing), GUE Cave 2 (which I didn't succeed in passing) and AG's whatever-you-want-to-call-it class that ended up with a NAUI Helitrox card. All three tested me to the max of what I could bring to the table as a diver, and I learned a lot from all three, although the experiences were not necessarily pleasant -- nor do I think that technical training SHOULD always be pleasant.
I've taken classes from PADI, NAUI, UTD, TDI and GUE; if you want hard, go GUE.
Useful runners up: My Puget Sound Marine Life ID class (not your usual one day PADI thing, but a distinctive specialty that involved four three-hour class nights and over 1000 slides) and Rescue.
Hardest classes: GUE Rec Triox (which I never succeeded in passing), GUE Cave 2 (which I didn't succeed in passing) and AG's whatever-you-want-to-call-it class that ended up with a NAUI Helitrox card. All three tested me to the max of what I could bring to the table as a diver, and I learned a lot from all three, although the experiences were not necessarily pleasant -- nor do I think that technical training SHOULD always be pleasant.
I've taken classes from PADI, NAUI, UTD, TDI and GUE; if you want hard, go GUE.