Helium, because I'm worth it.
Waterboarding at Guantanamo Bay sounded like a radical holiday opportunity until I looked it up.
As I recall, outside training, the only time I did donate was when my (single tank) buddy was indicating he'd reached the agreed gas level to ascend but I had plenty of gas and wanted a longer dive, even though it was a rather uninteresting drift. He was a little surprised and accepted the gas and we continued for a while longer before ascending. At the time he was working towards his sports diver and whenever the AS drill arose he turned into a stressed diver and went vertical and finning. Somehow the normality of receiving gas during this dive broke fixed that issue so everyone was happy.