I would tend to agree with WD Curry 111, if you're after full processing power, you can't beat Photoshop. I don't use a Mac, but the operability should be the same as with a PC.
Where I disagree, is for quick processing of personal shots.
I prefer Nero Photosnap. It's a free piece of software that's avialable for PC or Mac.
I camer across it by accident, years ago, and have used it ever since.
Operation is very simple and it allows you to adjust exposure, colour balance, brightness, contrast, etc. It has the ability to resize and crop, and has the obligatory special effects that seem standard nowadays.
As with most aplications like this, accessing, editing and saving is all very quick and easy.