This discussion always makes for an interesting thread.
My vote would go to the uPO, as By-Tor mentioned, it's simply too big. In fact, watches that are especially large and heavy end up being a kind of self-torture, as I love so many of their designs but can't wear them practically in many cases.
I would put the AP ROO in this category as well. I refuse to get rid of my MBK AP ROO because I like it so much, and yet it's not a watch that I typically can wear and forget about.
I'm not surprised at all that there would be people who are disappointed in the 1680/1665 MBK's. I would question whether the people responding had received "brand new" watches, which in the case of the vintage Rolex's, make them unappealing in an unmodded state.
I've had both unmodded and totally modded 1680/1665's and they are worlds apart. The unmodded ones I would not feel comfortable wearing out publically (seriously), and the modded ones would require a Rolex expert to crack the case backs to be able to distinguish them from the real thing.
By-Tor, I bet you if you had a genuine white dial 1680 with "the works" it would bring you around on the idea of the vintage pieces. What I've learned with the vintage pieces is that the dial makes ALL the difference. Combine it with a genuine bracelet and insert/crown/tube/crystal and it really does have the feel of the genuine.
I have a 1665 with the same specs as my 1680, the only difference being that the SD has what I perceive to be the best available "aftermarket" dial. It is damn good....much better than the MBK dials (even when modified properly like Palp did with yours), and still it bugs me.
My biggest disappointment would probably have to be the MBK modern sub. Not because it wasn't well finished, etc... but I can honestly say that if it weren't for the vintage 1:1 pieces, I wouldn't have a single Rolex rep in my collection. In fact, I wouldn't wear a lot of the semi-modded ones...but the fully modded ones are something different.
That's what makes me curious about the ceramic GMT II. I haven't been following the response to that, so maybe I'll have to take a look. I already have a "first generation" ceramic HBB, though, and at the new prices, I think they're actually a solid deal- I'd have a hard time imagining someone receive that watch and not being impressed.