Quick answer
Gracias
the everyday version
Muchas gracias
the alternative — see below for when
Which one to use
Gracias for everything, muchas gracias when you mean it. The reply is de nada — never no pasa nada, which answers an apology, not thanks.
All the versions
Gracias.Thanks.
Muchas gracias.Thank you very much.
Mil gracias.A thousand thanks.
Gracias por todo.Thanks for everything.
De nada.You’re welcome.
Where you are matters
Everywhere. In Mexico you will also hear no hay de qué and con gusto as replies.
Check yourself
Someone says “gracias”. You reply: