Ko Seung-Ji and Pan Da-Yang
These two may meet on a bit of a cliche, but their relationship grows from being business partners to friends and possibly more than that. They seem very real. They fight, they get along, they plan fun days together, they go through challenges and do what they can to overcome them.
I'm adding a best secondary couple here:
These two had a lot to learn about singing and performing from each other. Jason taught Pil-Suk to believe in herself and follow her talent. Pil-Suk taught Jason to respect other people's talent and appreciate working as a team player.