I collect firsts. I used to collect pretty broadly, but have lately narrowed down my buying to a couple of authors and a general area: genre fiction, mostly science fiction and "weird fiction" (think HP Lovecraft et al). I'm actually in the middle of a major purge, hoping to cut down from around 2,000 books to 500 or fewer.
My best acquisition, BTW, was a copy of Edward Abbey's "Down the River", first edition / first printing in dustjacket for $.25 (25 cents!). I eventually sold it to a dealer for $400, he in turn sold it to Edward Abbey's son, who had been estranged from his late father and was attempting to put together a collection of first editions of all of his father's books.
Best acquisition of anyone I know: a friend, who is a small time book dealer in Iowa, bought a box of books at a farm auction. One of the books turned out to be a first edition of the Federalist Papers (with both volumes), which is a highly sought after book. To cap it off, it had the signature of the original owner - a member of Congress from the Federalist Party! The signature was validated by research in the Library of Congress. The box of books cost $300 at the auction, but that one book had an estimated value of $75,000 or more.