There is no one that is "most" compatible. Standard advice: Stick to the Major brands. If you're thinking of shooting Raw make sure you're camera is supported. And for which ones are... We need to know your version of iphoto and the OS.
For instance: here's the current list...
For my money: best thing to do with DSLR is to handle it. Pick it up, hold it, put it down a few times. The size of your hand and the grip of the camera are not always a match.
DLSRs are either supported or not - if they are supported then they are supported and you need to choose the feature set you want
each version of the OS and of iPhoto has a different support list - here is the latest one for Lion through version 9.14 -- http://support.apple.com/kb/HT4757
My own personal choice is the Sony Alpha for many reasons -- I have the A55 and it is great -- others like Nikon, Canon, etc these are strictly personal choices - any supported camera is the same to iPhoto