Do you have a nice monitor already and maybe an SSD that could get some good use out of? Try a mac mini, the pov spec quad core is really good value (when you get the ssd in it too) and generally is a nice little choice.
If not get a retina macbook (the 13" looks amazing now I've seen it and the 15" is a beast but pretty expensive).
Imacs are always a good choice but it really seems the minimum is to order a fusion drive which makes them quite expensive, powerful though.
Anyway boils down to what you are planning on doing with it. Light/average gaming at 1920x1200 can be done easily on the mini (and the 13" retina), bit more gaming orentated I would push for the 15" retina or imac options with the dedicated gpus.