Formal:
* Je mérite mieux. (This is the most direct and literal translation.)
* Je suis digne de mieux. (This emphasizes your worthiness.)
* Je vaux mieux que ça. (This means "I am worth more than that.")
Informal:
* Je mérite mieux que ça. (This is the most common informal way to say it.)
* J'en mérite plus. (This means "I deserve more than this.")
* Je peux trouver mieux. (This means "I can find better.")
More nuanced:
* Je suis capable de mieux. (This means "I am capable of something better.")
* Je mérite plus de respect. (This means "I deserve more respect.")
* Je mérite quelqu'un qui m'apprécie. (This means "I deserve someone who appreciates me.")
The best option will depend on the specific situation and the person you are talking to.