I'd say Vodafone if you're after content. Vodafone Live! is teaming with stuff, rivalling the other accomplished 3G portal in the UK operated by 3.
In terms of data pricing for use of 3G bandwidth outside of the operator portals T-Mobile with Web n Walk is by far your best bet. Vodafone's national coverage is superior to T-Mobile's, however.
Three, 3, the first 3G network in the UK has been largely a joke. Personally I'd advise you to avoid them, but depending on your local coverage, your precise requirements and your ability to bear their chronic customer services operation they can't be completely ruled out.
You might find
Talk3G helpful in determining the best network for you.