Formal:
* Vous m'avez beaucoup aidé(e). (This is the most literal translation and is appropriate for formal situations.)
* Vous avez été d'une grande aide pour moi. (This means "You have been of great help to me" and is slightly more elaborate.)
Informal:
* Tu m'as beaucoup aidé(e). (This is the informal version of the first sentence, suitable for friends or family.)
* T'as vraiment été super pour moi. (This is a more casual and friendly expression, meaning "You've been really great to me.")
* Tu m'as beaucoup facilité la tâche. (This means "You made things a lot easier for me" and is a more specific way to express gratitude.)
You can also use the past participle of the verb "aider" (to help) to express appreciation:
* J'ai été beaucoup aidé(e) par vous. (This means "I was helped a lot by you.")
* J'ai beaucoup bénéficié de votre aide. (This means "I benefitted a lot from your help.")
The best choice depends on the specific situation and your relationship with the person you are speaking to.