Hello! « For me » is to express its effect on you or it's benefit for you, whether it's good or bad. « To me » is more to express opinion.

Hi, Seokho. Both statements are actually correct but it depends on your intent. If your intent is to express an opinion, you may use "it would be a paradise to me." If your intent of going to Jeju had an effect of a paradise or a wonder to you, you may use "it would be a paradise for me."