Te amo - I love you (I think this is the strongest)
Te quiero - I love you/ like you (literally I like you, but I think it has a stronger connotation than in English)
Te adoro - I adore you (is the literal translation)
Other ones to know:
Me gustas - I like you (I "like like" you, not quite love and more than the later)
Me caes bien - I like you, you do well by me