This is a loaded question. It all depends on what category of apps YOU enjoy, games, utilities, productivity, ...
Best thing to do is search the Market categories that you enjoy and try a few of the free versions.
I like to listen to Radio so I have Tune In and I like to listen to my channels on Pandora sometimes, and I like the game Freecell. I like to listen to books so I have Audible because I have an account with Audible.com.
It just depends on you.