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If white; I'd try 1. g4. For real. Since it'll probably take my opponent straight out of book, and I'm a very tactical player, so my best chances are when there are lost of pieces on the board and my opponent doesn't know what to do ^^
Or inverted black defence (attacking with your favorite defence, and an extra tempo, is, at least for me at club level, often successful).
If black; something you don't play alot, but you don't believe your opponent has much knowledge in. Ie not choosing the sicilian, but go for the latvian gambit (that's the name, right? 1. e4 e5 2. Nf3 f5) or something else that's suspicious ;).
Acctually, I don't know. I'd just try to deviate to unknown lines in my favorite defence as soon as possible.