All depends on how you're gonna use it and what $ you're willing to part with. You can go the cheap route (Radio shack and other no name brands) but you'll be spending a lot of time digging trying to uncover all the false hits they give. Better quality units are able to distinguish targets a bit better and can be adjusted to compensate for various conditions and items you're looking for.
I have a White's Surf Pro that I got for underwater searching while scuba diving. I'm pretty happy with it.
Check out:
http://www.whiteselectronics.com