I'll choose 2, one BG and one GG coz it's hard to decide
For girl groups, it's Rough by GFriend (2016). It just takes me on such an emotional ride, unlike any song. It doesn't sound overly sad like a ballad would, it's upbeat but also very nostalgic, makes you yearn for days gone by and the passion in the vocal delivery coupled with that instrumental really sets that mood perfectly. And when the guitar solo comes in, it's like the icing on the cake. Of course, having GFriend with that choreo helps too.
And for boy groups, it had to be The Chaser by Infinite (2012). Unlike Rough, which lets the instrumental come in and take over between verses, this one is basically a non stop romp from start to finish. The momentum never lets up, and even towards the end, it feels like the song is constantly building up. The only release you get is by hitting that replay button mercilessly again and again. It even makes me think of something cyberpunky- neon colored landscapes and bikes racing past you like in Akira lol. Either way, this is Sweetune's masterpiece.