Adobe Photoshop CS5 is the clear choice and I use it (fairly easy to use, very advanced and powerful too). There are also many in-depth tutorials on it and many users can help you with it.

However, sometimes if I want to do some simple effects with images which are difficult to do on Photoshop, I may use Microsoft Paint or sometimes even Microsoft Word 2010 (they have those cool image effects such as reflection)!

I don't do video editing, so I don't use Cinema 4D or anything like that.