Best phone? The phone doesn't mean a whole lot as long as it has the tethering feature and is 3G.
As far as using your phone instead of a data card, here's how it compared:
Data card:
$59.99 for 5GB per month
Separate contract required
3G speeds, where available
Dial-up speeds, where 3G is not available.
Quick to logon and start browsing.
Tethering phone:
$59.99 for 5GB per month
no separate contract required, add to your current wireless service
3G speeds where available
Dial-up speeds, where 3G is not available
Slower and harder to logon, with more errors
If you receive a call on the phone, you must answer it...internet will be terminated.
I am definitely not a fan of AT&T's 3G service and their tethering service.