Cocks, of course, but I'm not overly fixated on penises. I do do do like penises. My preference there is towards smaller, uncut members that point upwards, but I'm really not that picky. I maintain, however, that the focus on the penis is a primary cause of too much lackluster sex.
Look at it this way. It's just not that hard to make the penis stand up and take notice. And with too many guys, the penis takes over, they ejaculate right away, and then their bodies become unresponsive, and they lose interest in participating.
Working with the other parts requires more skill and subtlety (Usually, that is: we all know that guy who just wants you to grab his nuts and pull or bite his nipples harder. We like that guy, too, but we only want him occasionally.) and lasts longer.
As with most things, there's a combination of the material you have to work with and what you do with it. Some guys just have better, and more responsive, nipples than others. There are guys (like me, alas) with nipples that you could stroke and lick all day and elicit nothing more than a "how about that weather?" Fortunately, there are also men who start with prominent nipples that grow more prominent under prolonged attention. Men who writhe and shudder with delight when you've bitten down lightly and flicked your tongue over a nipple for a few minutes. That's the kind of guy you take home to your partner for a threeway. Or at least the kind of guy you brag about when your partner does something to piss you off.*
Few body parts (except, again, the penis) are as evidently responsive as the nipples. Asses can be perky (praise be to Cthulhu), but they generally don't get perkier just because you happen to have your tongue buried in them. Some men have very sensitive ears. But it's not that the ears themselves are responsive: it's that the entire body responds to what you're doing to the ear. The same is true if you're licking a guy's neck or biting his shoulder and his whole body tenses.
There's a fundamental difference between visual and visceral attraction. Or at least there is for me. I'm aware that a lot of guys are more visually oriented than I am and that the sight of a really nice ass might be enough to make them sprout wood. I have a very definite response to seeing a nice ass on a guy walking down the street, but it's a very different response from the response that I get when a guy kisses me or when I bite down on a guy's nipple. (That kiss/bite response, by the way, would be sproing.) I'm glad both that I get the visual thrill and that it doesn't give me public wood.
I didn't always have the visual thrill. Back in the days when I was working so hard at convincing myself that I was straight, I was pretty good at not letting my eyes linger too much on men. Women, for obvious reasons, weren't a problem. But now that I'm out and proud (if not loud), I finally understand how straight men feel about hooters. I am all about (discreetly) staring at asses and uttering the appreciative "damn!" To be clear, unless I'm alone in the car, the "damn!" is spoken internally, and since I have a relatively impassive face, no one knows. Also to be clear, the internal or external pronunciation is "DA-yum," with the smallest hint of a Southern accent. It's more appreciative that way.
As much as I love -- and I do, deeply love them -- the nips (And the lips: last week I sat through a tedious Melissa Joan Hart Christmas movie with my daughter because of Mario Lopez' lips. Mr. Lopez is undeniably pretty, but he really only makes my motor run when I can watch his lips move.) it's the ass that most ignites my visual pleasure. I would be hard pressed to choose, but I might slightly prefer gnawing on nips to tonguing the bung. But for viewing pleasure, a t-shirt stretched tightly over prominent nipples (which, again, I love dearly) will generally lose out to jeans-covered melons.
Fortunately, we live in a world where you can have both.
*I don't really condone bragging to your partner about guys you've done when you're pissed off at him. You should only brag to him when he's bragging to you.