When I'm making dirty martinis, I alternate between Stolichnaya and Chopin potato vodka. Chopin is from the same company that produces Belevedere. They have very different flavors.
I find that a lot of the ultra-premium vodka on the market is so over-filtered that it doesn't taste like anything. Grey Goose is the poster child for this. I can buy $7.00/1.75L plastic jug vodka and push it through a filter if I want vodka that doesn't taste like anything.
When I'm using vodka in mixed drinks, I tend to use mass market Smirnoff.