It depends on how much you want to spend on it. From what I've read, the Garmin 'Montana' is a really good one.
It has a large screen so it's easy to see without taking your eyes off the road too long.
However, even with it's shortcomings, since I can't spend that much, I use a Nuvi 500 and have found a way 'round most of those shortcomings. The biggest problem I've had with it is 're-routing', so i take the track I want to ride, convert it to an 'overlay map' and load into the navigator. This way, my route's always there and I can tell that the dam thing's re-routed.
If my wife ever upgrades my allowance, I'd like to get a Montana