What is the best Canon DSLR I can buy?

The short answer for highest image quality is the EOS-1Ds Mark III. Body only, it runs roughly $7000. For speed, it's going to be the EOS-1D Mark III which has a slightly smaller sensor but, shoots at 10 frames per second.

The long answer from this Canon shooter is, you may want to think twice about this decision. If it's a question of the Canond EOS-1Ds Mark III versus the Nikon D3x, neither is demonstrably superior to the other. If you already own a bunch of Nikon glass, even DX format glass, the D3x makes a lot more business sense.

For one thing, there's the cost of replacing all your lenses with Canon gear after recoving, at best, only 80% of your investment in Nikon glass. We can add to that the fact that Nikon has recently caught up to, possibly even edged past, Canon in terms of image sensor technology. With that said, I would still confidently say they are generally equals.

Only the Canon 5D Mark II is likely to offer something Nikon doesn't yet offer in their pro/semi-pro level, full-frame bodies. That single advantage is HD video recording. However, Nikon beat Canon to the punch even in that department with the significantly cheaper D90. Nikon has basically replaced all their lower end cameras at this point so I would expect a successor to the D3x or D700 that offers video within the next 18-months.

I'm not saying all this because I am planning to to switch to Nikon...I'm firmly planted on the Canon side. But, if you area approaching this as a business, you should realize that unless the ROI on your new gear is 100% or better, there's little point in buying it. I've shot both Canon and Nikon gear for years and it is nearly impossible to argue any economic advantage to swtiching from one system to another.

If you were any good of a photographer, you would know that the camera is only 50% of the great photographer. But a good midrange camera and some really good lenses first, spend the money and take a photography class instead. The best camera in the world is useless unless you understand photography as an art form. Plus, assuming you're newer to photography or else you wouldn't be asking this question, you probably won't understand all the features or how to use it. But in a short answer, the most expensive Canon DSLR would be the Canon 1d Mark 3. There are two versions the 1ds Mk3 or the 1d Mk3. Each has a different use.