My top 3 :
My Mister
Our Beloved Summer
Itaewon Class
So, Twenty Five Twenty One was there until ... ... I watched ... ... My Mister. Amazing show. Hard to even compare it to anything. It stands alone like a beacon in the light daring any other kdrama to knock it down. Complex, philosophically deep and full of likeable characters. Not your usual formulaic kdrama either. If you like stuff like Backstreet Rookie or Our Beloved Summer or other pure romance, then this isn't that. It does have a whole bunch of everything though and don't get me wrong, I also like the romance kdramas. Love them actually.
Anyways, no wonder it is the highest rated kdrama on IMDb sitting @ a 9.1/10.
I cannot more highly recommend this show. If you haven't watched, put it next in queue. You'll come back here and say thanks for the rec.
Check my custom lists to see what I've watched and how I've rated them to get a gauge on how I rate my stuff.