Based on my experience for the past few months, if you don't care about delivery speed and are in US:
For ADV, Media Blaster, Central Park Media titles:
www.digitaleyes.net -- 35% off plus 10% off coupon = 41.5% off
For Geneon/Pioneer, Bandai titles (Preorder 30 days advance):
www.rightstuf.com -- 25% off plus 10% off coupon plus GotAnime discount = 39.25% off (best scenario, usually at 38% off)
For Geneon/Pioneer, Bandai titles (Non-preorder)
www.digitaleyes.net -- 26% off plus 10% off coupon = 34.1% off
Few catches, Rightstuf requires order to be over $99 to get free shipping and Digital Eyes requires $20(or $25?) to qualify for free shipping so you will have to plan ahead before ordering. Also, the 10% off coupon are usually dollar off coupon like 5 off 50 or 10 off 100 so you basically have to order several DVDs together to get the discount.
Of course, studio sales at Rightstuf can save you quite a lot if you can wait for the twice a year event. Usually you get 41.5 to 46% off for ADV, MB, CPM and 39.5% off for Geneon/Pioneer and Bandai titles.
Bundles in the Rightstuf weekly special can be quite a money-saver as well. Usually you can get ADV titles for over 60% off for the whole series and recently there are several Geneon/Pioneer series sold at similar discount. Example? How about Cardcaptor Sakura Vol. 1-9 at 67% off?