My favorite is S. gigantea. Once acclimated, they are very hardy. Though not a carpet anemone, H. magnifica is another good option. It's the anemone seen at the beginning of Finding Nemo. Both species are natural hosts for both percula and ocellaris clownfish.
The other two clownfish hosting carpet species -- S. mertensii and S. haddoni, and nice options but each has its drawbacks. S. mertensii tends to grow very big and typically lays flat in the rockwork, taking up a lot of real estate. While small, they may be somewhat colorful, but most turn a drab brown color. S. haddoni are sand dwellers, so you'll need a deep sand bed to make it feel comfortable. They can also get very large.