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What are the forums for internet startup founders?
(I hope if I remove the word best it is not argumentative anymore? Please do not close this question unless you add an answer here)
Hacker News: http://news.ycombinator.com is one.
Try the StackExchange OnStartups site, or TheFunded (registration and clearance required).
StartUps are a particular subject strenth on Quora too it seems.