Best data plan (individual user): AT&T
This is a pretty subjective one. There was a time when AT&T had flat pricing, one size fits all. They did away with that with the introduction of the iPhone 4, instead going to a tiered model. So now you can get 200MB/mo for $15, 2GB/mo for $25 or 4G tethering for $45 for 4GB, $10/GB after that.
Verizon has ended its $30/month unlimited data plan in early July four different tiered programs: $10 a month for 75 megabytes, $30 for 2 gigabytes, $50 for 5 gigabytes and $80 for 10 gigabytes. Going over your monthly limit will cost you $10 per gigabyte.
T-Mobile doesn't separate talk/text and data in their plans, nor do they have an unlimited plan, so I can't give you a dollar rate. Their data rates start at 200MB per month and top out at 10GB. The 10GB plan with unlimited minutes is the most expensive, at $119 a month.
Sprint sure makes it easy: Unlimited everything for $99.99 a month. For lower rate plans, there's the Data Pack for $15 and the Pro Pack, with more features, for $30. But starting at the beginning of 2011, it added a Premium Data Add-on fee of $10.
AT&T offers variety and since it's the only network where you can talk and surf at the same time, it wins this category.