Formal:
* Je vous apprécie beaucoup. (This is very formal and can be used for someone you respect but don't know well)
* J'ai beaucoup d'affection pour vous. (This is a more formal way of saying you have feelings for someone)
Informal:
* Je t'aime beaucoup. (This means "I love you a lot" and is used with someone you are close to)
* Tu me plais beaucoup. (This is a direct way of saying "I like you" and is more casual)
* J'ai un gros crush sur toi. (This is a playful way of saying you have a crush on someone)
* Je te trouve très sympathique. (This means "I find you very nice" and is a polite way to express your liking)
The best choice will depend on your relationship with the person you are talking to.
Remember that French culture is often more formal than English culture, so it's always a good idea to err on the side of caution and use a more formal phrase unless you are very close to the person.