The Crawford V2 is the one of the best ones out there. The biggest thing to ask is "What are you going to do with your car?" If you plan to track your car (making high G corners), it has been discussed at length that the Crawford unit works best. If you just plan on normal DD activities, the GS unit is very nice and will work well for less money. If you do decide on a Crawford unit, I purchased mine from Circuit Motorsports. They were the cheapest place on the web that I found.
Disclaimer - I have NO affiliation with Circuit, just trying to pass along the savings I found after an exhaustive search.