Imo the best mouse to upgrade to is a BenQ Zowie FK2. Been wanting one for awhile. But the Deathadder is such a great mouse that I just haven't needed another one.
The Steelseries Rival 310 is also really good, but it feels cheaper made than the deathadder. You can find one of those pretty easily for around $35, where as the Zowie FK2 is about $60.