Formal:
* Eu gosto de você. (This is the most literal translation and is appropriate for formal situations.)
Informal:
* Eu gosto de ti. (This is a more informal version, usually used with friends or family.)
* Eu gosto muito de você/ti. (This emphasizes your liking and can be used in both formal and informal situations.)
* Você é legal/gostoso/bonito/bonita. (These are all informal ways to say "you are cool/attractive" and can be used to express your liking.)
Romantic:
* Eu gosto muito de você. (This can be used romantically, but it's important to be aware of the context.)
* Eu estou apaixonado(a) por você. (This means "I am in love with you" and is a more serious expression of affection.)
Remember that the best way to express your feelings depends on the specific situation and your relationship with the person you are talking to.