Where is the best online design store?  

I run retrosixty.co.uk based...
I run retrosixty.co.uk based in the UK. Also happy to discuss payment plans with clients.

for vintage I just tried...
for vintage I just tried modern50 they a have a really nice selection of interesting products good prices and have a really good contact. Plus by discussing prices a bit I've had a 70$ discount on two eames side shells... I'm now waiting for them to be delivered... we'll see then if thay are as good as they seem to be! the link is:
http://www.modern50.com/

www.sofasale.co.uk
Based in Manchester UK
Selling Modern Famous name furnishings.
Canceled Orders, End of Line, Surplus Stock, at a fraction of the retail price.
http://www.sofasale.co.uk

Europe
in Europe, this one is nice but never bought from them:
www.owo.it
nice little shop and great service (realy honest seller, upfront about delivery times etc, if it takes longer - offers discount): www.concona.de
good looking shop with nice selection but horrible shopping experience (14 weeks wait, account charged, never reply to emails and never ship) http://www.ambientedirect.com (Europe+US)
http://www.concona.de

highbrow
I really like www.highbrowfurniture.com they have a great selection and free shipping

All depends...
if you like unique pieces/or quite rare things and have pots of wonga! and dont mind waiting for gorgeous bits of gorgeousness?,try the boys at the following link.Or...for more mundane stuff (yuk, i know we all need it) what about Heals?.
http://www.originindesign.com

Modern 50 is definitely a...
Modern 50 is definitely a great site, I recieved my chairs and they are great plus the shipping was ultra fast!! I really recommend this site for its products, friendly contact and overall professionnalism.
www.modern50.com

and the ...
friendly contact is made more friendly in the link box
http://www.modern50.com

Thought of another....
These guys are fantastic for vintage pieces as well as current design.Ask to speak to Owen ..
http://www.twentytwentyone.com

Doesn't get much more comprehensive...
than this in the Southern Hemisphere...
http://www.matisse.co.nz

These guys have a great...
These guys have a great website, and prices seem fair as well to ship worldwide: http://www.poaa.nl/index.php

Support the small guy
Support small businesses wherever you can. You guys have a lot of power to vote with your pocketbook.