Hmmm, I'd say it depends which aircraft you get on. Air Canada don't have PTV's in economy so if thats your bag then avoid them. Northwest DC10s are to be avoided but their A330s are great. Likewise United 777s are ok but their 767/747s less so. From your list I'd say Continental as you are pretty much guaranteed a new aircraft with PTVs (apart a very few thin routes operated by 757s) where as with the others you may get a dirty old aircraft.
It really depends what your criteria is. If you can get on an NW
A330 then go for that, otherwise CO
in my opinion.
But then again UA
do have ch9 ATC ...