Anyone who says Vegas is best probably has never been outta the country. It's highly overrated, after 3 days you get bored, even if it's your first time. Amsterdam has rude, stuffy people. The weed and hookers kind of make up for it, but you won't be doing that 24 hours a day, unfortunately.
That's why I say Prague. Good beer, good people, the best architecture of the three, and probably the cheapest.