It's hard to give an unequivocal "best" here; more what scratches the itch.
Samsung has the better screen and faster hardware, so if you want a bit of hardware futureproofing, go that route. I like the Desire's build quality and its custom UI elements more, though; HTC Sense is a bit more powerful. That and HTC has been much better about supporting Android devices over time than Samsung (some devices even a few months old were left without upgrades despite clearly being fast enough).
