Most cost efficient air conditioner?

Question:Which is the most cost efficient..a window/wall air conditioner that circulates air from outside or a split system that circulates inside air?

Answers:
That all depends upon what you want your a/c to do.
Are you wanting to cool just one room?
If so, then a packaged wall or window unit is probably the most cost effective way to go.
Also window/wall units don't necessarily circulate air from the outside unless they have an economizer built into them.
If you are wanting to cool several rooms or your whole home I highly recommend a split system that uses the newer R-410A(Puron) refrigerant rather than the R-22 refrigerant, simply because R-22 is going to be phased out in several years like R-12 was. Several years down the road after R-22 is no longer manufactured and is worth more than gold(like R-12 is now) you don't want to have a service technician come out to your home, because your a/c is low on refrigerant, and tell you that it will cost a thousand bucks to fix and recharge your system; the bulk of the cost coming from the charge of refrigerant needed to get your system up and running again.
Also if you want a quieter, more efficient system you will definitely want to go with a split system that has a higher SEER (seasonal energy efficiency rating)rating number; typically this would be in the range of 16-18 or higher. Systems that have a lower SEER rating 12-14 tend to have noisier compressors and take more electricity to run.
You will pay more for a system that is more efficient but in the long run it will save you more money.
With window/ wall units as with split systems the more money you pay the quiter and more efficient the unit.
If you do get a more efficient system find out if your local utility, electricity company offers rebates on the installation of such systems.
I hope that helps.