It could be for me my A700/Mk25 scubapro regulator set. It is just a wow experience going to that from the rental reg experience.
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@ScubaDocER: I'm guessing that you aren't a mask-on-forehead kinda guy.
Stuff that I'm really glad I bought: HP100 tanks, drysuit, BP/W, Apeks regs, Suunto mosquito, my Canon A570IS