In the UK I like:

www.wiggle.co.uk online, and www.mud-dock.co.uk in person.

Wiggle has a great range of products and really good service.

Mud Dock is where I bought my Cannondale, and they've got a great policy of 4 free services per year for the lifetime of the bike (which is nice). I've found that they've been really great to my bike, and haven't been patronising to me. They're also very proactive in the Bristol area with trail-building and sponsoring events.