A TLR for sure (such as Rollei or Yashica) based on personal experience. Good results and people love it. Old people reminisce, young people assume credibility.
The 120 SLRs can look and sound cool. I havent' owned 120 SLRs, but I'd agree the Hasselblads look the nicest and have a pro reputation (despite the fact that pretty much all 120 SLRs were made for pros)
The folders can be pretty darn cool and take good photos too. The oldest ones look cool but can be difficult to focus for short depth of field as they have scale focusing. Something rangefinder folder can be also cool. The fuji gf670 looks modern and quaint at the same time. A shame it's so expensive. certo6 is a popular ebay seller for old and oldish folders.
Funny story--I took this thing to a local historical event to shoot a few snaps...everyone was looking at me like I had three heads. Even the people dressed up like it was 1890 were looking at me like I was crazy LOL. Weirdest thing. Wonderful camera though, and the IQ is outstanding.
My other vote would be for the Kodak Medalist. Only gripe with this one is having to respool onto 620. Other than that, an outstanding camera that would double as a hammer if you put it on the end of a stick.
It's a very unconventional camera. I found out about it reading a book on Linhof equipment and I thought it was extremely cool. I have never seen one in person and it is my understanding that they're quite rare.
Alpa 12TC is beautiful, I can't afford it, so I have the Fotoman 69, which is almost as cool.
The Fujifilm GF670 I think is a modern masterpiece, to be in 2012 and a company is still making a folder medium format camera is amazing. And at $1600 new, it's fraction of the price of a new Leica film camera.
I've never used one, but I think the Walker large format cameras look great, a traditional camera made from modern materials is very interesting.They're not stupid expensive either.