What country are you in? All the major carriers in the US support the S3 and I think they are all $10/month + taxes and fees.
On Verizon, it shares the data plan of your phone, although on your account it is configured as a separate line linked to your main line.
Not sure how you define as "the best". For me it is all about coverage and for me that means AT&T. I have tried T-Mobile and Verizon and in my area they don't have the same coverage that AT&T provides. I did not test Sprint since I don't know anybody that has Sprint in my area.
If you want the cheapest watch plan here in the USA, then you might want to consider one of the current T-Mobile plans that charge 'one' rate where fees and taxes are included in the monthly fee. On T-Mobile, the $10 watch wearable plan would be $10 and includes all taxes and fees.