Here are a few other ways to say "I'm flattered" in Spanish:
* "Me siento halagado"
* "Es un honor para mí"
* "Estoy muy agradecido"
* "Muchas gracias"
* "De verdad"
* "No sabes cuánto me alegro de oír eso"
The phrase you choose will depend on the context of the situation and the level of formality you wish to use.