Fun thread. But I can't tell whether "cool" is measured by reactions from others, or our own feelings of "coolness" when using them. Based on the previous responses, it seems like people react to big (as in, "What the heck is that??" -- Mamiya RB or Pentax 67 fit that bill) or retro (TLRs). And if they're shiny, and/or have stuff hanging off them like a finder, all the better.
I did recently see a guy using (actually using!) a gold-trimmed Hasselblad 501C. I thought he was pretty cool for not letting that sit locked away at home, but I don't know whether the average person on the street would have noticed particularly.