This question gets asked several times per week and therefore you should not expect to receive many replies to your question. Use the search function to search for previously posted threads and make sure to take a peek at this one: http://forums.remote-exploit.org/showthread.php?t=2191
My personal advice, the Alfa 500Mw AWUS036H.



i got my alfa few days back and i must say.... i was stupid to think about prices "oh my that too expensive!" so i ended with cheap adapter first and modding it to have better range. haha
