Arrival Time is my preference. The faster it shows up, the faster I can get civilians out. Load time is so short as to be inconsequential, really. (EDIT: As was pointed out in the comments) Load Time becomes more important if you increase the helicopter capacity by research - if you don't have faster load times, it may not fill entirely before lifting off.
Reload Time is what I've found to be most useful, as it lets me fire more. That said, without good Impact Time, it's hart to accurately target. I keep these close to even, so it can shoot again as soon as the shells land.
I'd say Range or Reload Time. I must admit, I rarely use this enough to level it significantly.
Accuracy has always been my first choice. It stops more zeds, and if you structure your forces right, you end up funneling most of the to your infantry, so they don't have to move much.
Refire Time, definitely. Position them well at the start (I like a corner, with a target point across the map) and they can cover as many as half of the zed entry points. This is wonderful when combined with artillery strikes on those areas. Faster firing means more dead zed, and that leads to faster leveling of your snipers, and fewer victory points for the bad guys.