As others have said, the guy attached is the most important.
I like a cock clean, and trimmed is nice. I don't like hair on the shaft (a little is fine, but the less the better). A nice, sexy curve, and a plump head is the best. I like the ridge around the head to stick out at least 1/3 centimeter more than the shaft... it's sexy to look at, and feels good, too. And veins, oh they're sexy, they add such definition, such visual texture. I like smooth coloring with dark, raised veins, and a richly red head. And what I love most of all is the rock hard feeling of a throbbing cock, with satin smooth skin... it feels so good to caress, to lick and taste... my lips, tongue, and fingers are the most sensitive parts of my body to detail, and while my pussy gets the most actual pleasure, the rest of me looooves to rub up against a sexy cock, enjoying every little vein and ridge.
I hate piercings, personally. I think that a cock is the epitome of natural male beauty, and piercings destroy that.
To me, though, the issues are, in order of most important to least, cleanliness, standout thick head, rock hardness, prominent veins, hair management, then last, size. Not too big that it won't fit in my mouth, but a good size nonetheless.
And I'm not sure if it belongs here... but smokers suck. Gagging is a rarity for me during blowjobs, but smoker's cum tastes so incredibly bad, that I won't touch a smoker. And, smoking tends to reduce a guy's ability to get as hard, because of circulation issues, as I recall, so that's 2 counts against it.
All in all, though, I love all cock, so even if it's not perfect, it's still a delight to play with, to lick, to touch, to ride... so nice!