It all depends on how much you want to spend.
Basic, but still good...Garmin 2610. A lot of people really like this unit.
Or, go all out, spend the $$$ one time and be done with it.
Garmin Zumo 550.
I went Zumo 550 route.
I STRONGLY second the Zumo 550 route. It has bluetooth, so it can link up with a phone and headset and manage your calls off of the GPS unit, so that you can talk while you ride (not that it's recommended but for a quick call to tell someone you're running late or something, it's great). It's also a touch-screen.
Get all GPS units cheaper at GPS Discount, plus great service.I wanted a Zumo 550 but the price was way up there, especially after you add the XM antenna.
Someone reccomended the Garmin 2610 refurbished unit so i ended up getting it. It is a really nice unit for under $200. I'm very happy with it.
I got mine from the link above, very good service.