MacPro is better, just because it's a much more powerful machine. If you're wondering about simple day to day use which is better, the iMac would probably be better just because it doesn't cost nearly as much and offers enough performance for what most people do.
Well this is kind of an Apples to Oranges comparison. The MacBookPro is a powerful and beautiful laptop and the iMac is a desktop computer. Just depends on what you need and what you are willing to pay for.
I'm going to get a MacBookPro myself next year.
Mac OS 10.6.7 - Personal Ubuntu Server 11.04 - Server
According to me imac is best because iMac with Intel Core i3, i5 and i7 processors offer speeds between 2.93 GHz and 3.6 GHz with up to 16 GB of 1333 MHz faster memory, discrete ATI Radeon HD craphics map.