This question has been answered many times on the more traditional "hardware" sites (Tom's Hardware, etc), as well as on the nVidia discussion boards. The people who maintain the pages at Wikipedia also do a fantastic job of summarizing the specs of the numerous nVidia cards, as well.
The best I can come up with based on comparing these cards is "it depends on what you are looking to do". Just like any other design / purchasing decision, there are a lot of factors to consider that only you can determine the answer to. Broadly speaking, the GeForce/GTX line is aimed at the gamer market, the Quadro at professional desktop applications, and the Tesla at compute-intensive work. While they do share common components and architecture, there are differences among the cards, and there is not one "right" answer.
