Any stone, ceramic, or porcelain tile would be great.
They each have their own negative and positive aspects, but as far as the heated floor performance, they are the same.
Porcelain is the most durable. Stone (granite, marble, slate) is beautiful, but does require bi-yearly sealing due to its porous nature. Ceramics will be the least expensive and can be just as beautiful as any stone or porcelain.
For more information:
You should read GreenHomeGuide's "Buyer's Guide to Green Floor Materials."