Michelin Pro2 Race in the Summer, Michelin Carbon in the winter/spring. I've been riding the new Krylion carbon this year and it's just as excellent a training tire as the regular carbon.
We do ride Conti GP 4-Seasons on the tandem... we need a heavy duty 700x28 for that. If Michelin made the Krylion in a 700x28 they'd be on the tandem too.
DrPete