Well, Victor took away part of my thunder with his pick of Texas Hold 'Em, but I have three other frequently-used iPhone apps in mind.
Twitterrific is my first choice. I love Twitter and formerly used the awe-inspiring Hahlo web app on my iPhone, but there's no native Hahlo app...yet. So for the time being, Twitterrific is what I use to blast my tweets out to the world. If native Hahlo ever appears, Twitterrific gets nixed immediately. So much for loyalty, eh?
My second fave is DataCase. Yeah, I've tried a bunch of the others, but DataCase is still (in my opinion) the best little app for shooting files to my iPhone and then sucking them down to another Mac. I've even had it work with Windows. The interface is not my favorite, but I can put up with a lot for functionality.
Número tres is NetNewsWire. This is the iPhone companion to NewsGator (Mac) and Feed Demon (Windows). Although NetNewsWire received low reviews in the iTunes App Store, I've found it to be very useful for my RSS needs and have no complaints. And the price is right - FREE!
App Store Links --
Twitterrific (free, US$9.99 for an ad-free version)