"Best" compact flash card

I've had really good luck with Sandisk Ultra II cards.

A good mix of performance and value.

I'd tend to go for either Sandisk or Lexar. Mostly Sandisk Ultra II, although whether you need something faster is determined by your camera - my 400D does uses some of the extra speed in the Extreme IV range, but not much of it - my 10d doesn't even come close to stressing the UltraII.

7dayshop is probably cheapest at the moment, although Amazon may have something - with memory cards I'd only buy from real Amazon, rather than their z-shops and linked 3rd party suppliers though as there are a lot of fakes floating around at the moment.

Any good named brand is fine.
I've used Integral, Sandisk and Fulifilm.

I recently bought a Kingston CF card from 7dayshop, it arrived in just the little protective case, all the OE packaging was missing. I returned it and got my money back. Probably a genuine card, but it was not marked as unboxed on their site and I won't accept unsealed CF cards.

Warehouse express are good as are http://www.mouse2house.co.uk/

Another vote for Sandisk Ultra 2, got mine from Play.com cheap enough. Worked at -15/5700metres without problems. Unlike miy fingers!

I noticed the Jessops Cards say Sandisk on the back and they are cheap as well.

Another vote for Sandisk Ultra. I've not had any problems with them.
If you're buying from Amazon watch out for exorbitant postage costs. One Amazon associated seller was charging x2 lots of £5.00 for two xd cards. I didn't buy them!