I've seen a couple of okay-looking penises, but only a couple of fabulous ones. Fabulous. Proportioned, as Squishor has described, but the head about the same size as the shaft (I don't like the head to be too big or too small). Balls also in proportion to the penis. (I've seen some balls that have hung so low that it looked like the guy's sac was straining to hang on to them. That was distracting.) Generally pointing outward, not noticeably down or up... that's limiting to the kinds of positions that work. Not too veiny looking, except for maybe one big one to show that the member has substance
. But on that point, even the most fabulous penis I've seen had its days where a lot of veins were visible. Oh yeah, and not too squishy. Some of them are a little squishy even when they're erect.
Gilda's right, too. When I'm going to be seeing a lot of a particular penis, I like to make friends with it first... so it's better if it's playful. And nice to pet and kiss. My vagina and one of those fabulous penises were fast friends and shared a lot of late-night conversations.
EDIT: Almost forgot about the thickness. In general, the shaft is widest at the base. I've seen one that got skinnier at the base and it looked like a corndog. That was weird. I just kept thinking to myself, "He'd better get some engineers in there to stabilize that shit before it falls off."