Answer:
Vodka and gin seem to give you less of a hangover than any of the other liquors. Also, the "top shelf" brands (the expensive brands) tend to give you less of a hangover. I would suggest Grey Goose vodka or Bombay Sapphire gin.
Also, if you drink plenty of water before you go to bed (at least 32 ounces, which is two very large glasses) it will help prevent (or at least diminish) a hangover.
You can also mix the vodka or gin with club soda or tonic water, so you'll end up drinking even more water.