iPhone iPhone iPhone!!!
I'm currently resisting the temptation to buy the Navigon satnav software - its £50 (I know, but it makes your phone a real turn by turn guided satnav), BUT...
TOMTOM are releasing one VERY SOON, and are also releasing a dock for the car that has an extra built in GPS receiver for impeccable accuracy.
I will get one of these two apps, I just cant decide which one!!
I'd say this highlights why the iphone is the best by a mile - I'm fairly sure that what you see is what you get with the nokia's etc, whereas you can keep adding and adding to the iphone!