There is no single best solution right now. Tom Tom's pretty cool. It's got big buttons for keys, and a smooth scrolling 3D view. Routing seems to be top notch also. It's got a cool detour feature where you can say to avoid the next mile or so of your current route. iGuidance only lets you avoid a whole leg of a route. Finally, Tom Tom supports walking/cycling routes.
However iGuidance will tell you if your destination address is on the left or right. It also has ATMs in the point of interest. Finally, when on the freeway, it will tell you to get on the 405 South on the right. Tom Tom doesn't do that, which is a pain. You have to pay close attention to the 3D view at onramps.
What I'm trying to say is, if you're looking for the perfect software, you're not going to find it. If you're mostly going to use it for driving, I'd recommend iGuidance. This is the app I use all the time.
So, even though