I've used a Garmin, TomTom, the built in GPS in our Mercury, and my phones GPS.
TomTom wins.
The Garmin was too complicated for a lot of people. I didn't have a problem with it, but it was kind of a pain because it took so many clicks for a simple task, which the tomtom could do in like 3 presses. Simplicity is nice when you're trying to adjust stuff while driving. (Probably not safe, but hey, you WILL do it and when you do, it should be easy!)
The built in mercury GPS was stupid, calculated dumb routes, the voice was annoying and she'd constantly give you a big long speech about how the map data is incomplete whenever you got off a main highway. VERY annoying. Plus since it was bulit into the car they had some stupid safety thing so you had to pull over and put it in park to pick a destination.
My phone is great, but it's my phone and the GPS software just isn't quite as good as tomtom. I put my phone in a close second.
TomTom has plenty of voice options, so you can have an authoritative sounding british guy giving you directions. The routes are fast and great, if you encounter a road block you can tell the tom tom to go around it in 2 button presses. It's just good.
Another thing the TomTom does, which I don't recall the others doing, is it tells you which lane to stay in. So if the highway is gonna split off in a few hundred yards and you gotta stay in the left lane to remain on route, the TomTom will tell you to stay in the left lane. Instead of just yelling at you to get back on track when you are in the right lane instead. (Yeah road signs do this for you, but it's a nice feature anyway.)