The highest quality one you can get is probably one of the ProClip mounts. They consist of two parts: a mount which is specific for your vehicle and usually clips into the dash (very securely) as well as a holder for your specific device which screws into the mount. This combination will give you a very stable mount on your dash, but it is pretty pricey. Most of the mounts are around $30 and the device holders range from $25 to $70 or more, depending on features (whether adjustable, whether it has a passthrough for your dock cable, etc.). So you're looking at anywhere from $55 to $100 or more depending on the type of mount you want.
There may be other good quality dash mounts out there, but this is just one in particular that I am familiar with.
They have a bunch of option and are about a third of the price of the proclip. Plus they work with a case on the phone which is a huge decision maker for me. They also have a bunch of mounting options.