Christiania vodka is the best IMHO. definitely worth a try.
It's a 6 times distilled potato vodka from sweeden.
Thors Hammer vodka was the smoothest I have ever tasted, but I don't think they make it anymore.
Really, the only time you need a super gourmet vodka is when drinking martini's. Other than that, Skyy, Absolut, Ketel One, or Stoli is more than good enough. It's funny when people order a tall greyhound or seabreeze, then comment,
"ahhhh that's good vodka."
Once you mix it with anything, it doesn't matter. Yes it may be worth it to drink very good vodka while mixing if it does not give you a hangover because it has less impurities. But don't mix vodka because of the taste.
Heck, even a Martini is 1/3 water. Once you get done shaking it it's pretty diluted by that point.
I went to a bar in San Francisco where they porured the vodka from the bottle straight from the deep freeze, with a big chunk of ice around the bottle. Still very cold, but pure vodka, no dillution. I could definitely tell the difference.
Tapakeg
http://www.christianiavodka.com/