Here's a short list of tween libraries I'm quite familiar with:
- Tweener
- GTween
- TweenLite
- Eaze
And, here's a link to a nice comparison of a suite of different libraries for both AS2 and AS3: http://www.greensock.com/tweening-speed-test/
Ultimately, it's going to come down to which syntax you prefer and which one you're most comfortable with. I suggest you look a few of them over, pick one or two and code up an easy comparison to see for yourself...
Good luck!