Right now I'd either buy a 27" iMac or a Mac Mini, unless you really want a laptop in which case I'd get a MacBook Pro. In any case try to get an i7 processor if you can and buy extra RAM separately
to install yourself (it's easy).
Right now I would not buy a 21" iMac or a Mac Pro (although next year when they get a good update I want to buy a Mac Pro), and I would not recommend a low end laptop for Reason.
Right now I'm using one of the latest model Mac Minis with an i7 processor and I put in 16 gigs of RAM myself. My only complaint with it really is that the Intel HD 4000 it uses for video is a bit on the low end for some games or other heavy graphics work, but for Reason it's working well.
If you care about high graphics performance or you want a nice display with it, I'd definitely go with the 27" iMac. You'll also get better performance for a longer time since it's got a faster processor.