My favorite is the Patch tool. You can hide or blend anything using it! I used it at work all the time! It give ladies their perfect skins and guys that clean look!
It would be difficult to just name one tool. I think Photoshop is amazing. I just watched an interesting tutorial tonight on layer masks. I altered this image to practice. I made a layer mask and used a black brush to hide what was in the frame on that back wall. Then I brought in this picture (of the woman), re-sized it, and placed her to fit right into the frame on a lower layer. Also, I wanted to experiment with the HDR toning, so I did, and adjusted the hue/saturation. I've been learning Photoshop for about 3 or 4 months. I think it's so much fun! I really like it. I'll also be learning Illustrator and InDesign soon. I would also like to experiment more with making my own brushes and actions...
I'm still new to Photoshop, but I love the ability to mask layers and the tools I like the most so far are the brushes and burn tool.
Do you mean in Extended version or just CS6. I think the 3d options are great so far, and maybe will be better in CS7.
I love polygonal lasso tool,because you can select a thing with that and clean other places sorry for my English !
Photoshop is amazing..... Its just like a game to me.... Although i like everytool but in my opinion pen tool is the best , because i like editing pictures....