Well, if it seems obvious that he's absolutely alone, I'd probably go for the legs and then eat the entire body. Or whatever zombies do these days, do they still stick to a diet of guts and brains or something else, I don't even know.
If it's likely that there are other survivors nearby, I'd go for the dick. Why? It's vulgar, humorous, and very demoralizing for all the onlookers. And if the survivor escapes and decides to bandage it up and meet other survivors, you get the classic horror movie situation where someone who's been infected hides it and hopes that it doesn't work... and that generally fucks up the survivors. And what is a better place for hard-to-detect infectious bites then thighs and, you know, sexual organs?
That's like, helping build up drama.