Forrester evaluated the 10 largest UK retail banking brands: Barclays, Halifax, HSBC, first direct, Lloyds TSB, Nationwide, NatWest, Santander, The Co-operative and Yorkshire Bank.
"The Co-operative Bank tops our review largely because it supports potential customers looking to research and buy a current account at every stage of the process," said Forrester analyst Vanessa Niemeyer.
Using its own Web Site Functionality Benchmark, Forrester's top five were:
1. The Co-operative Bank
5. Lloyds TSB
Blogging about the report, Forrester vice president and research director Benjamin Ensor said: "Overall many of the UK banks' sites aren't giving prospective customers enough help, particularly when it comes to comparing different accounts."
He added: "Designing web sites well isn't easy, particularly when you have to deal with the limitations of legacy systems.
"By failing to support customers with the content and functionality they need, banks risk forcing customers to go elsewhere - as happens with the comparison sites that bombard Britons with TV ads every day," added Ensor.
The Forrester report, called Benchmarking UK Banks' Sales Sites, 2010, showed that almost 60 per cent of British internet users had researched a financial product online in the past 12 months, more than any other European country.
Author of the report Niemeyer said: "Considering that UK customers are among the most likely customers anywhere to research and buy financial products online, the nation's retail banks have been oddly slow to develop world-class web sites."
The report also found that 40 per cent of online Brits had applied for a financial product online in the past year, double the European average.