While I only have experience on UA and NZ I thought I would give you my feedback on those two. Have heard and read some good things about QA.

I have experienced the NZ Business Priemer on a recent flight from AKL-LAX (and NRT-CHC) and was not impressed. The flight attendents were friendly and did a nice job however not any more so than UA. I was disapointed in the level of service for a ~13 hour flight - meal service was quick and snack selection was poor. Food quality was on par with UA but was presented in a better format - nice dishes and food looked great. The seats make traveling with a companion next to impossible and being that they are angled toward the isle you are unable to look out the window (admitively there ain't much to see out the window but I like to day dream). To turn your seat into a bed and vice versa you need to get out of the chair. I like to lounge or lie down and switch often - can't do that with the new seats. Also I am 6'3 and found the otoman which become part of the bed to be annoying, there is a seat belt on the otaman that can be used for a guest but it keep getting in the way of my feet. The in-flight entertainment was great lots of choices and bets UA hands down. I mean no offense to my friends down under NZ is a great airline but I was not a fan of the new service.

I will admit that I am a UA fan so there is a lot of things with UA that I may overlook. First the seat - you can look out the window and sit with a partner unlike NZ. I like the UA C seat as there are a lot of options in how you position it. If you check out the UA forum you will find that a lot of fokes like the UA C seat better than some of the slanted "lie flat" C beds. If the seat had just a little more recline and a little more lift of the footrest it would be fantastic. UA IFE lacks the competition - no VOD and only 9 channels of video that replays over and over, if the FA remembers to reset it. I do think think they have a nice selection of audio channels plus Channel 9. Food seems to be on par with NZ but lags slightly in terms of presentation (menus not as nice or plates). UA service - can be fantastic or not. This is my one issue with UA - they are not always consistant. When you get a great crew they are as good as BA, SQ, etc. But when you get a crew that disappears for the whole flight thats when I get disappointed - dosen't happen often but when it dose its memorable.

I have heard the QA seat is very comfortable but as far as I know its one of those slanted lie flats so it depends if you like that. Lots of little features like water bottle holder. Good VOD on par with NZ as I have heard. Food reviews seem to be good.

My order of preference for a great experience in C would be:

1. QA (based on what I have heard, I of course would take my feedback as heresay as it isn't first hand experience).
2. UA
3. NZ

I don't think you'll nessecary go wrong with any of these choices.