Both them are good and they have same meaning(most). I think 제일 is more used but both them are good. and you can say 'favorite' '제일 좋아하는' '가장 좋아하는'. and your sentence is correct and you can say '제가 한국에 있었을 때 가장 맛있었던 음식은 비빔밥이에요.' they are present form. If you want to use past form, '한국에서 가장 맛있었던 음식은 비빔밥이었어요''제가 한국에 있었을 때 가장 밌었던 음식은 비빔밥이었어요'