You can say "이 중에서 뭐가제일 맛있어요?" without omiting '가' when you want someone recommend the most delicious food to you.

Thank you :D

postposition is make sure about 사과 is subject or object or predicate. we difficult to learn korean postposition but maybe english is incomplete language to korean.

이 and 가 is something like IS. you know theres no postposition in english. im not sure im right because my korean teacher is not korean. but korean is perfect language. i learned likethis: in english: to eat apple, say to epple, apple said, apple is. you see, just change the order. but in korean: 사과를 먹다, 사과에게 말하다, 사과가 말했다, 사과는. not change order only form is change. sorry for my english but i did my best haha

No, we wouldn't say 뭐 맛있어요. :)