+1 for Nalgene, but the GSI ones are probably fine.
Since
Nalgenes are the de facto standard, a lot of bottle carriers and other accesories are made to fit Nalgene bottles. A lot of water filters, if you intend to filter for purification, are threaded to screw
directly onto Nalgene wide mouth openings. I've actually gone
more toward Nalgene because so many other products work with Nalgenes, but they're a great bottle, period.
Nalgenes are very leak proof which is a huge deal if you plan to do cold wx outings and want to take a hot water bottle with you into your sleeping bag.
One item of note: with the polycarbonate (clear plastic) versions, there is some controversy that chemicals leach into the water. Supposedly the translucent bottles made out of polyethelene (HDPE) are safer.
http://www.nalgene-outdoor.com/store/detail.aspx?ID=59 You'll also notice that the HDPE are $6.60 vs. $9.00 for the polycarbs.
One (kinda) bad thing about the colorized polycarb bottles is that sometimes the color from iodine water purification gets distorted whereas with the white translucent bottles, I can always see the color which is just a double check to know that the "dosage" is right.
Unless your need is immediate,
shop around. I've seen very good deals ($5.00 for the polycarbs, $4.50 for the HDPE's) on line.
Addendum: Karst sports has the half liter HDPE Nalgenes on sale for
$1.62!!
http://www.karstsports.com/na16wimohdbo.html