This a very individual preference. What are you looking for? What is it you
If you like music, give Pandora a try (if it works on your carrier and blackberry model). You'll find it on blackbbery app world, or google it.
I also like Bing. I think it is a good, basic, all purpose app that most people would want.
Browse the aftermarket forum here at blackberryforums. Also see the software list at BlackBerry FAQ - BlackBerryFAQ