I've had to think about this. I assume we are limited to hotels that we have actually stayed. My vote goes to the now closed Hotel Bora Bora, with Lizard Island and Amanpulo a close second/third.

If we are a keeping score, could you issue a negative vote to
Raffles (tourist trap extraordinaire)
Park Hyatt Tokyo (famous bar with an awkward hotel attached)

While both of these places might be cool to 'check out', they leave much to be desired as 5* hotels.