lets see...
#1- the nameless blue bottles I find hiking. very strong, easy to flake, and beautiful.
#2. Ale 8 bottles(green glass)-Not nearly as strong, but harder to flake and more durable.
#3-clear 4 sided glass wine or some other drink bottles(I don't care what b/c I don't drink it
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)- beautiful for knives, and really sharp... flakes very well!
I can get an almost unlimited amount of bottle bottoms from hiking and my neighbor(he makes beer, and gives me his reject bottles)- flatter bottoms aren't always better IMO.