There's no doubt that PhotoShop and PaintShopPro would rule the roost as far as advanced technical manipulations of digital images are required. Unfortunately, both are just a li'l too techie for normal users ... requiring quite a steep learning curve. As some others have posted here, any normal creative user would spend more time trying to understand the various controls than actually being able to apply the various effects. For high-quality editing and image manipulation for the print and other media requirements, they would be the programs to go with.
But for normal run-of-the-mill image editing, printing, resizing, cropping, creating thumbnails, aggregating images (with or without captions and filenames) on single/multiple contact sheets, creating slideshows, batch-file operations, image cropping, file-size and image-density changes, titling or inserting text in images, applying various effects, etc. the FREE program IrfanView really takes the cake in my opinion.
I have been using it over 10 years, initially as a image editor and also as my primary audio/video player (yes, it does that too ... very well). The main program was initially quite small, and didn't hog memory (like PhotoShop, CorelDraw, or PaintShopPro would do). You also need to install it's various other plug-ins (available separately as well as in entirety) for other specific usages you may have (like QuickTime, PostScript, JPM, MrSID, etc). Most of its functions are activated through intuitive single key-presses, like T for thumbnails, B for batch programming, S to save, L & R to rotate left or right, V and H for vertical and horizontal flips, etc.
There are many other programs out there, but none really offer the massively easy and intuitive image manipulations that IrfanView offers. Try it out to believe it. Further, it can play most audio and video streams as good as (if not better sounding) than the top leaders (WinAmp, WinMediaPlayer, RealPlayer, etc) too.
The only aspect of image-manipulation IrfanView currently lacks is the concept of applying and manipulating layers (like in PhotoShop). This is really required in advanced image manipulations. For this alone, one may free download Paint.net, which resembles the Windows built-in Paint program and combines the same Paint look-and-feel with the layers functionality of PhotoShop et al.