I would go for the Nalgene bottles if you're looking for ultimate leak-proofness. Beware there are imitation bottles out there now, and they might work fine, but I don't have any first hand experience with them.

What I can say is my 1L Nalgene (the smoke-colored Lexan ones) bottles have been everywhere and I've never had a leak. Not so much as a drop. Definitely worth the money for quality and peace of mind.

http://www.nalgene-outdoor.com/store/subcategory.asp?categorysubcategorycode=5

Prices seem to run in the $7-9 range per bottle, depending on where you order or buy them from. Sometimes the outdoors stores (REI, EMS Sports, Sierra Trading Post, etc.) will have sales and you can snag the bottles for $4-5.