If image quality is your priority and you don't plan to shoot in the driving rain, surf or a dust storm, save your money and get the 5D Mark II. The 5DmkII has the newest and best image processor as well as the highest ISO capability. It's also the lightest and smallest of the three camera bodies but, you can add a vertical/battery grip if you prefer. The one area of major disappointment on the 5D is that Canon chose to use the AF system of the XXD series cameras rather than that of the 1D/1Ds. That said, I use my 5DmkII primarily for architectural/landscape photography and I'm beginning to shoot special events...As of yet, I haven't found the AF system to be much of a hinderance.
Now, the 1DsmkII and 1DsmkIII are full-spec, professional grade cameras. They have longer lasting shutters and fully sealed bodies. They offer somewhat more ergonomic controls but the key difference is the greater freedom these cameras offer for customizing the configuration of those controls. AF point grouping and expansion are also options available with these cameras that aren't possible with any other Canon except new 1D Mark IV and 7D (it is hoped the next 5D will offer a similar AF system).
The current 1DsmkIII shoots one frame per second faster but, if super high resolution photos is your focus, then speed is not likely to be a major concern. It should be noted the price of the 1DsmkIII has fallen dramatically since the introduction of the 5DmkII and Nikon's D3x. These cameras seriously undermine the value of the 1DsmkIII and Canon was forced to reduce its original $7995 asking price to around $6100 with current rebates. The one significant advantage this camera can't claim to offer is better image quality than the 5DmkII. I've had the chance to shoot a 1Ds several times over the last couple years. It's a great camera and a joy to shoot with but, none of the purported advantages this camera offers over my 5DmkII justify its price. Your opinion may differ.
I haven't mentioned the 1DsmkII because its the oldest of the bunch. If you need the 1Ds series build quality or just prefer the control layout, this camera makes a lot of sense. To me, it makes a lot more sense than paying $6k for the current 1DsmkIII. That said, it is hard to ignore the 5DmkII offers higher ISO capability and the higher resolution offers more freedom for cropping in post-production. Another significant issue with these models is that most have seen heavy use and have more than 100,000 cycles on their shutters. This means you are more likely to find one whose shutter needs to be replaced and the repair will not be under warranty. Shutter replacement costs about $500.