"...how long is a piece of string...?"
Yes, that is the kind of question you've asked.
I would be willing to bet that for virtually any app in the repos, somebody here uses and swears by it. Others may use it, but swear AT it. And still others don't use it at all.
And, of course, it also depends on the desktop environment you use. A Gnome user would probably have decidedly different favorites than a KDE user, for instance.
Still, some generalizations might be made.
OpenOffice is, arguably, the best office suite.
Firefox for web browsing.
The Gimp for image editing.
Digikam for photos.
VLC, Smplayer, and Dragon seem popular for watching videos.