I have two different ones in my cars:
1. Valentine 1 - if you don't want one installed, this is probably the best bet. costs 400, but it will tell you how many signals, does front and rear (through the windows), and seems to really avoid falsing. A great unit that really hasn't changed in about 15 years, and still usually wins all the reviews. as a bonus there is a great rear view mirror mount/wiring kit you can pick up so you don't have to stick it to windshield - and the mount is jump proof
2. Passport 9500ci - this is an installed unit, and very pricey at about 1500 (install extra), but has all the bells and whistles - it diffuses laser (actually saw it working, got pulled over and cop couldn't give me a ticket), does front and rear radar (doesn't tell you if front or rear though), handles red light camera locations, has GPS, will tell you your actual speed.
I like both of those, albeit at wildly different price points.