Punch - especially the male second lead.
Nobunago Concerto - Shibasaki Ko's Female lead
Okitegami Kyouko no Bibouroku - Aragaki Yui's Female Lead
Orange Days - Shibasaki Ko's Female lead
Quartet - All four of them
Goodbye to Goodbye - especially Chae Si Ra and Jo Bo Ah's characters
Miss Korea the second leads, played by Lee Seong Min and Song Seon Mi
Pretty much any one of Lee Yo Won's lead characters - she picks REALLY well.
As a general rule, I often find much more interesting characterisation in secondary characters. They may or may not have a "loveline", but it's precisely because of not being constrained by the formulaic expectations/demands of lead roles that (good) writers often invest their seconds with layered, nuanced characters more "real" than the leads.