Spirytus Rektyfikowany is the strongest Vodka,This Polish Vodka is a murderous 96% ABV
This is probably the strongest drink you can buy, but it's a "rectified spirit" so not technically vodka. Similarly Everclear is 95% but classed as a "grain spirit"
There's a bit of controversy over what the actual definition of vodka is, but it is normally a grain spirit that has been filtered at least once, and then a small amount of water is normally added. The filtration is what distinguishes it from the rectified or grain spirits.
The strongest actual vodkas I've seen are Pincer Shanghai Strength vodka - 88.8%, and Balkan 176 - 88%.
Haven't tried the Pincer but the Balkan 176 is dangerous stuff, pretty hard to drink neat because it burns your mouth so much!
If you want a pleasant tasting, reasonably strong vodka that is smooth enough to drink neat, I would recommend Stolichnaya Blue Label. It's 50% ABV but is actually very nice drunk straight from the freezer, unlike some of these other crazy drinks which are just brutal unless you mix them with something.