I do not really like police dramas but tend to love them only if an undercover role is given. So can anyone recommend a drama where the main lead plays an undercover role, it could be either side meaning the police can go undercover with the criminals or the criminal can go undercover with the police. I also like it when the main lead doubts himself. Cruel City would be a good example of what I am looking for.

Also the drama itself doesnt have to be a police drama. City Hunter is also something I enjoyed and while it isnt a typical police drama it still involves aspects of going undercover.

Lastly dramas with secret identities. Although this isnt really considered going undercover it still has to do with hiding who you really are with another identity, hence why some undercover dramas deal with people with secret identities. Examples of this would be, Bridal Mask, Healer, Iljimae and of course the two dramas I mentioned before, both Cruel City and City Hunter.

Hidden Identity - all about undercover   police work
Nirvana in Fire   - the main character hides his identity in order to extract revenge
Double Face (2ep jdrama) - cop works   undercover  for yakuza and v/v
Ghost - hacker assumes role of policeman
Return of Iljimae - kinda like Iljimae but not really
Time Between Dog and Wolf -  cop works undercover for gangster

Movies:
New World
Infernal Affairs
Joker Game
Secret Reunion
Secretly, Greatly

Other than the awesome ones Juls recommended:

Criminal/Police/Investigation Ones:
Ouroboros
Joker Yurusarezaru Sousakan
Pinocchio
That Winter, The Wind Blows
The Man in the Mask
Runway Cop
Angry Mom
She's on Duty
Vampire Prosecutor (hides the fact that he is a vampire)
Gap Dong
Opas: The Series Season 1 (hides the fact that he can see ghosts)

Non-Criminal/Police/Investigation Hidden Identity/True Character Dramas:
High School King of Savvy
Ma Boy
Me too, Flower!
Fall in Love With Me
I Love Lee Tae Ri
Runaway Sweetheart

Bad guys - not totally undercover but the inmates get realised from prison to solves crimes. Its totally badass and brilliant. Even if not into crime dramas, you will give it a 10!

@Juls you recommended some interesting ones, i think ill try a few of them but Ghost stands out the most.

@bossmama the jdrama you suggested seems good plus it has one of my favorite japanese actors in it.

@pigtails4ever theres alot there some ive seen but some i havent. i think ill try Opas but im not really into thai dramas, but since you have other good suggestions ill definitely give it a try.

@oppalover87 ive watched a bit of it and its pretty good, i like the older cop in that show. he also played a role in city hunter and played it off well. hes a really good actor.
Hmm....May Queen has some aspects of hidden identity.  

Swallow the Sun also has the hidden identity.

Also Innocent Man and Beyond the Clouds.


@Dreamwalker havent watched may queen but i am watching swallow the sun and innocent man. beyond the clouds got a little boring to me once he started to fall for the girl. so i think ill give may queen a try.
Yea, I noticed you were watching scandal, and May Queen was another good drama with the same lead.
I second Angry Mom, the undercover part is hilarious ^^
Hong Kong Drama:

Emergency Unit (E.U)
Lives Of Omission