depends on what level of quality/sophistication you want and how long you are willing to wait.
I would say build one yourself if you can solder (
or learn how to), there's good guides on
Jamma Nation X (and tons elsewhere...), it's the cheapest option and you learn something.
If you want to buy, the
Omega is the new boss on the block. Or find someone to do a consolization for you (like Xian Xi of Jamma Nation X, he is a wizard and can build one to your specs, even cheaper than an Omega - but he gets a lot of requests and you'll have to wait LONG if you can get on the list at all). Stay clear of Neotropolis (ultra overpriced but good service) and Analogue (rumored to be bad quality builds, expensive, technical knowledge leeched from the community without giving anything back).
Emulation has also become great in the last years, a real alternative if you just want to get your game on. Or think about getting a NGCD...
