No way to pick just one:
Adjustment Layers - they save my butt over and over if I want to further correct something without actually altering the layers below (or attached to it).
Masking - couldn't do without it for compositing!
Healing Brush/Spot Healig Brush (new to CS2) I use the most for dust and artifacts.
Highlight/Shadow Adjustment (Image>Adjustments>Highlight/Shadow), copy to a new layer with Soft Light mode - lower Saturation, and adjust Levels to taste.
Actions - saves truckloads of time - especially for repeated mundane tasks.
Layer Modes - unlimited control, especially used with correcting adjustments.
Levels and Curves - quick and broad color and density correction, then fine-tuned color adjustment (respectively).
Pen Tool for clean selections and illustration.

Sadly, haven't gotten the Match Color thing to work right so I just balance everything by eye as close as I can.Hope that helps-
-Lew ;-)