Deffinitely 7-zip. I used to use WinRAR but since I discovered 7-zip I sticked with it.
It's free , fast , light on the memory , plenty of features , and great compression. Also it has no silly nag messages.
WinRAR and 7-zip are more or less the same. They do the same thing comparatively well. WinRAR looks a bit prettier and 7-zip is free. Since their performance, interface and features are more or less the same, 7-zip is the way to go. WinRAR makes .rar files and 7-zip makes .7z files but they can both extract the other's native encoding (7-zip can extract .rar files, even segmented ones, and WinRAR can do the same to .7z files) and they both can create and extract .zip files. They also both can password-encrypt, auto-segment, create self-extract archives and integrate themselves into the Windows shell the same way. They also both have x32 and x64 versions (7-zip x64 came before WinRAR x64). All in all, there's literally no reason to pay for WinRAR unless you like the "prettiness" of it. Personally, I could not care less.