Without a doubt Eddy Curry.
He was one of the biggest reasons for the downfall of the Knicks. First off, We traded our first round draft pick to Chicago for Eddy Curry. We would've gotten the second spot, which was used for LaMarcus Aldridge (who was soon traded from Chicago to Portland).
So, we basically traded the chance to draft LaMarcus Aldridge for Eddy Curry. Using this pick, we could've also been able to acquire Brandon Roy, Rudy Gay or Randy Foye, if we didn't want to take Aldridge.
But no, we took Curry. He performed very poorly, playing no defense and not rebounding. He was plagued with injuries after the first two seasons. Curry stayed out of shape, so he could get payed without having to actually play. He succesfully wasted the team's money.
Then, he brought more negative attention to the team when he was accused by his chaffeur of some really disgusting stuff (I will not go into it, but you can read about it online).
Today, he continues to not play and waste our investment in him. He currently remains our biggest financial obstacle in acquiring talented free agents for 2010.
He is not only the worst Knicks player of the last decade, but the worst NBA player of the last decade.