This is easy:
Can lights - my 18 and my 21. Gotta have a buddy light. I think I've sold more can lights by having the buddy light... friggen Salvo, and Sartek and Halcyon and Dive Rite - these people owe me, big time.
A can light just makes every dive more enjoyable.
Wrist Gauges - liberation from the brick on a stick was the second best move I made.
Spring Straps - why on earth would anyone dive with rubber straps these days? Talk about horse and buggy. I totally don't get it.
I love all my gear, but these three things I love a little bit more than the rest.
At the very top of the list (just moved there this weekend, BTW) is my Nikon D70 photo rig. 99% of SB doesn't get to see my photos, as I only post them in the SoCal room. But I've learned more about the marine environment I dive in, and more about the creatures, their interactions, their interdependance, their amazing lives (and their names, for heaven's sake) from taking thousands of Macro pix over the last 13 months than I did over the previous 7 years of diving. I appreciate more, I am in greater awe and I enjoy diving by a greater order of magnitude as a direct result of taking shots and then studying the shots and essentially reverse engineering all the stuff in there.
I'd say my camera rig has been the piece of equipment I regret the least.
---
Ken