It'd probably be some form of benzodiazepine. However there are so many weird and wonderful substances that I have yet to experiment with, so my answer is biased.
I'd be down to earth and choose a benzo because if I had an infinite supply I wouldn't have to worry about anything, ever again.
But if I wanted to choose something for recreational value, it'd probably be a potent opioid or an upper... MDMA wouldn't be bad though. Infact it would be very, very good. %)