Tile, carpet and timber are all suitable floor coverings. There are a few design considerations to be adhered to :
Tile: use adhesive suitable for underfloor heating
Timber: use timber that has been kiln dried to approx 8%.
Carpet: always use a good quality thin carpet and a good quality underlay.
Always consult your flooring provider before you lay a finished floor on top. They will provide you with the best advice on choosing a floor covering most suitable for underfloor heating.