From the three prams you mention, I would immediately discard one - the Nunas. Those are the bulkiest, the weakest, frame-wise, and the shortest in their reversible seat unit, which means the Nuna Mixx or Nuna Mixx Next will last you worst over time - ad will be the least practical.
So the final battle, at least from my point of view, would be between the Uppababy and the Bugaboo brands. Both are a very nice choice here, though, both are not the smallest after folding. My own personal opinion would be - go for the Bugaboo as it is LIGHTER. A lighter stroller, if you don't need a true all-terrain, off-road pushchair, means better maneuverability, easier loading in/out of the car, and overall, less fuss. The downside of it is that it is slightly lower to the ground, compared to the Vista, and tall parents usually prefer higher stroller systems. It is not low, however, and the handlebar extends high up enough, so I only would take that as a dealbreaker if you truly feel you need the tallest stroller possible.
One more thing. If you plan a second child soon after the first (in one, 1.5 years after), the Uppabby might be a better choice as it converts into a double stroller. But I wouldn't get it "just in case" if you truly don't plan it that way - as my own opinion is that lighter is better when needing an everyday stroller and not an off-road horse.