You can also use more elaborate expressions depending on the context:
* Σε ευχαριστώ πολύ (se efharistó polí): Thank you very much (literally "I thank you very much"). This adds emphasis.
* Σ' ευχαριστώ (s' efharistó): A more informal and affectionate thank you, often used with friends and family. The 's' is a contracted form of "σε" (se - to you).
The best choice depends on your relationship with the person and the situation. `Efharistó` is perfectly acceptable in almost any situation.