The problem is that what someone else has done is not always the best for you. The best sex act for a birthday present is either something they love but don't want to ask you for often or else something that they've really wanted to try and they're not quite confident enough to bring it up to you.

In general, oral sex for a long time with no expectation of reciprocation is usually a good bet. Combine that with a long erotic and sensual massage and you've got something that's usually a smash hit for a woman. For a guy usually straight oral or anal works, but some like to be pampered.

My boyfriend calls it his birthday blow job, and it's a really long blow job that doesn't lead to sex before he finishes. Of course, sex comes later, but it's not part of the initial act.

I had a girlfriend in college that let me have anal sex with her for my 19th birthday. Now that is a present that you don't forget.

I personally think planned birthday sex is a bad idea. There is already a bunch of pressure to have a good time on your birthday, and that kind of pressure doesn't make for good sex. Sometimes things go wrong, or someone isn't in the mood. Planning a specific act for a birthday, seems contrived, and likely end in disappointment.