I've been critical for substantiated reasons about Discover and their competitiveness with regards to credit lines, APRs and backroom operations. But their website is one of the best if not the best for the cards I have.
The Discover site is easy to use, well layed out and responsive. By contrast I was just on Citi's website which apparently they've reengineered over the past month and their's is horrible (but since Citi has given me a credit line 3 x that of Discover and an APR more than half as much, Citi is still the preferred card).
Discover's website is very good and so is Chase's. Bank of America and Amex is pretty good, with HSBC/CapOne and Citi following the pack.