I've seen a lot of kdramas so far. Almost all of the dramas in the Popular List. By now, this is my favorite drama. Mad Dog is sooo underrated. Disclaimer : I'm a picky watcher, okay, I don't rate 10/10 easily....
Mad Dog is one of the best dramas out there, it really needs more recognition. Why? Well...

Character development and acting skills is SO good!!! It really gets me goosebumps, especially Woo Do Hwan and Yoo Ji Tae's acting skills!
The storyline/plot is amazingly original and there's very much suspense / thriller in this. It really gets you thrilled for the upcoming episodes(at least, me. It did start off slow though). Not predictable like so many other dramas. Background stories of all main leads are pretty clear and well thought of.
Prettiest OST out there, Before The Sunset by Eric Nam. Honestly I still play the OST daily on Spotify lol. It's really emotional and fits every scene that you hear it with.

Filming/special effects are amazing. This drama is filmed very good, I can notice. Very nice scenery.
Only downside is some plotholes. Things that don't really make sense and the storyline could be a bit stronger with some more real "story" to it. But the acting made it really good.

The romance is not really important in here, if you're looking for cliche romance then best not be here. There's some romance in here but no multiple kissing scenes, not like that. Ofcourse, the genres are Crime/Investigation etcetera.

Absolutely LOVE it! I've seen so many dramas (DOTS, Healer, K2, Goblin, Strong Woman Do Bong Soon, W, Suspicious Partner, While You Were Sleeping etc etc), this one is the BEST I've seen so far! Props to all the actors and congrats on winning awards!