What is the best way to tell you Partner "I Love You"

Just look at her/his eyes straightly and smoothly, then just say "I love you", and describe how much you care about her/him. So much precious her/him for you...

In my opinion, saying these three important words to the one who you love face to face is the most direct and best way

Perhaps there is no best way to do it because people have different preferences. Anyway, let me give it a shot. For me the best way to tell my partner "I Love You" is to make him feel accepted for everything he is especially if he has unpleasant past. Example my boyfriend asked me if I would still accept him if he had a son from his previous relationship. I told him that his past does not affect my love for him. I still love him like a brand new person.