Interesting question! We have no clue to differentiate these meanings only with this context! Probably Japanese tend not to make meanings clear. So you can simply interpret this as "I like you very very much."
However, my personal opinion, I would choose the latter interpretation "I like you more than anyone else who likes you", if I were to be asked this question from my girlfriend. That's because it tells I am the best lover of her. The former one "I like you more than I like anyone else" sounds selfish a little because it could give impressions like 'I am making a decision of mate' or 'I have another person who I like as much as you'. "You're the best lovee by me" sounds disgusting, right?