I've always liked Photoshop. Everytime I've tried the gimp I've really struggled with the UI. It never seemed like something I could just open and start playing with.
Last year I played around with Krita by the KDE people. Excellent application.... it feels much more like photoshop. I just hope they've sorted the manual a bit since then. Trying to do some more advanced techniques did become difficult simply because there was no documentation/youtube videos!