Goodreader is great for me. I am in technical sales and have all of our reference documents loaded on my iPad for use at a moments notice. I almost use my device like a mobile digital print out, I bought this to play with but find myself using it for work constantly now. Weatherbug and the built in email app are very good for me too.
Honestly, it's a toss up between MLB.tv, AirVideo and Netflix. All three work very well and I like the fact that I can watch pretty much all of my media over 3G without sacrificing too much quality.