The Kuhmo SPT's are great if you are on a budget.

I've also heard good things about the Vredensteins.

I currently have Nitto Invos, and they were a big improvement over the stock Conti's, and also quite reasonably priced (<$800 for all four with shipping). They really smoothed out the ride, and are much grippier. Excellent day to day driver tire, but not the best high performance pick since the softer sidewalls can flex a bit much at high G's. Not so good in the ice and snow, but great in the rain - two huge sipes to displace the water.