Gamechanger on IPAD and IPHONE, is what we use. Stores the stats where you can get to them from your PC. Fantastic amount of stats. Last time I looked it was still a free app.
IScore is the easiest I have seen - especially on the Ipad. I have been trying to get the twitter functionality to work - and have not yet been successful. Still working on it!
I recentlypurchased an ipad and iscore. I have not used it in a live game yet so I can't speak for how good it is. It was a little difficult to enter the roster and getting a test game going but I'll keep playing with it to get used to it.
I used to use Turbo Stats and a Palm Pilot. It was great and easy to use. the only down side was the screen is so small and with my aging eyes it was difficult to see :)
For those of us that have Android phones try 6-4-3 Baseball Scorecard. It's very easy to start a game without adding names in beforehand which is one of my pet peeves. It also has the feel of using a pencil and a scorebook which is pretty cool.
iScore is very easy and provides a ton of stats. You can customize it however you want and the support from the designers is some of the best I ever seen. They monitor their forum regularly. Lots of different ways to output the stats. You can also sign up for the website and the stats are automatically posted to the site after the game. Parents or family members who are not at the game can follow along play-by-play if the scoring is done on an iphone.