Wow, what a difficult question; I really had to think about this one for a while. There's a lot of variables when I think about the question, such as: Which would I attempt (right now) given the chance; which peak most appeals to me, and which summit would I be most pleased to stand upon.
To answer each of those questions in order, I think the most appealing (right now) would be Cho Oyu. I would choose Cho Oyu because it's a good mountain to get some 8000m experience without as many of the objective hazards that a lot of the other peaks have, and it's relatively easy to access, making it a good excursion for those who don't have an extra three months of vacation time lying around. It also has the best success rate and best overall safety record of the 8000m peaks.
The peak that is most appealing to me is Everest, as I have dreamed of standing at the highest spot on Earth for quite some time.
With those answers aside, the reason I chose K2 is because it's the mountain I would be most pleased to stand upon. It's the most technical of the 8000m peaks, partly due to its incredibly inclimate weather; It's less traveled (to this date, there have only been about 300 ascents of its summit, compared to the estimated 3000 Everest has had), and it's the second highest mountain in the world. There are obvious drawbacks of this peak, like the 26% mortality rate in those who attempt to climb this mountain, compared to the less than 10% on Everest; however, given the chance (if I had the proper climbing resume already under my belt) this would be the crown jewel of climbs.