The Economist did a piece on this in the spring (in fact, I think someone even started a thread about it). Apparently it is the UK, which is frankly no huge surprise.

FWIW and to be slightly pedantic, most citizens of most countries do not have the right to travel visa free to most other countries; rather, they are able to apply for a visa upon arrival which in practice is rarely ever refused. So, for instance, it is quite easy for UK passportholders to enter Kenya by paying $50 at the airport in Nairobi and applying for a visa on the spot, but a visa is still required. There are actually very few countries (mainly within the EU) that most UK passportholders can visit truly on a visa-free basis.