The Altima isn't fun to drive, and its reliability is questionable. I'd forget about that one.
Among BMWs, the 3-Series looks considerably more reliable than the 5-Series in our stats, and has about as much space inside as the GTI. It's also more fun to drive than the 5. The 2013 on have been reliable so far. 2012 has become iffy. Overall German cars have become acceptable or better within the warranty period, then worse afterwards.
You seem to be focusing on V6s. But even with a four the Mazda6 is pretty quick. For a combination of driving enjoyment and reliability, it's probably your best bet among midsize sedans. If you don't need a large rear seat, the new Mazda3 is even more fun to drive.
I haven't yet driven the new GTI, and it's of course far too new to know how reliable it will be. In our stats, recent GTIs have tended to be good for the first two years, then worsen to so-so afterwards. Absolute horror stories are mostly a thing of the past, though there are probably still a few with the latest cars.
Finally, returning to Nissan, the Infiniti G37 might provide what you're looking for. They're not quite as involving as a 3-Series, but have a much better record of reliability, and tend to be quite affordable as used cars.