Well, the good old "get laid in ..." still works fine, as well as plain textlinks, although the CTR on some flash ads is pretty nice as well, especially the flash/geo ones from fling at 320x250 "Finally, a cure for the horny".
Really depends on your layout, the only things I'd avoid is any static banners. No idea why so many sponsors still offer them, every lame textlink gets more clicks than a banner.
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