So out of all of those, I admit that I've only seen Healer--so take my recommendation to see it with a grain of salt.
Healer utilizes all of its characters brilliantly, the acting is magnificent, and there is plenty of "mystery/thriller/action" in there. It's one of the most well balanced and executed kdramas I've ever seen (there's definitely a reason why it's rated so high).
I have seen cruel city and healer. In my opinion healer is a good drama with action but i think is predictable. you can tell what is going to happend in the end. cruel city is about drugs,money and power. i started it and i couldn't stop, i wanted to see what is gonna happend in the end. if you wanna see something "dark" and "underground" then you have to see cruel city.
Healer was awesome. A few minor inconsistancies near the end but I found them irrevelent and still rated it a 10.
God's Gift was so extremely frustrating to me. On one hand, it had a lot of suspense that kept me on the edge of my seat, along with some really awesome moments. On the other hand, there were so many plot holes and things that made zero sense to me. It was like two completely seperate sets of writers were alternating writing the script. Half of it was really great but the other half was awful and it was mixed altogether throughout. I ended up rating it a 5 simply because I felt it was exactly even in terms of awesomeness and stupidity. :p
I watched all of these and I liked God's Gift the best. It's an amazing drama that you'll love all of the characters and shere their pain as well :) Plus is the shortest of these haha
Figured out the mystery of Healer at episode 8 couldnt fifnish it tbh I think its highly overrated Cruel City is quite good mainly because of the male lead haven't seen 14 days but right now Im watching Vampire Preosecutor...I know it may not be your cup of tea if you dont like the whole vampire thing but it mainly focuses on the detective/Mystery element of it and it's an OCN drama so its full of action and crime
