Age of Youth (2016) - has multiple main characters and a few couples
Fight My Way (2017) - has a steady side couple
The Reply series has really great side couples, although these dramas are more about friendship than romance. Also, they are recent (2012-2015) but they are set in the past so I'm not sure if you would be interested lol
Reply 1997
Reply 1988
Reply 1994
^I third Descendants of the Sun