I've watched so many dramas but a very few have exceptional chemistry between the lead actors and that is what makes a drama GREAT for me...I really need some good recommendations in which the leads just seem to click with each other... hope you know what I'm saying... :P

The ones I've watched that I think have real good chemistry between the actors are:

Healer, Boys over flowers, My love from another star, Secret Garden, Descendants of the Sun (watching it currently), Coffee Prince, Dream High, My name is Kim Sam Soon, Pinocchio...

Reply 1997...
I'm kind of biased though as I just finished it and am still on a high xD But my god the leads had so much chemistry!
Also Kill Me Heal Me, I felt had good chemistry :)
Witch's Romance and Bromance. Main leads in both have amazing chemistry with proper kiss scenes. 
Secret, Ugly Alert, Marriage Not Dating, Discovery of Romance, 
hana yori dango, innocent man, full house (thai), the princess' man, bromance, d-day, mary stayed out all night, gu family book
Can We Get Married?
Fated to Love you
Witch's Romance - must watch
I Need Romance - must watch
Reply 1997
The Master's Sun
Autumn's Concerto
Bromance
Empress Ki
Full House (Thailand)
Noble, My Love
Secret
The Princess' Man
I'm one of those viewers who can't stand the awkward, juvenile interactions between characters who are supposed to be lovers. I.E:

  1. Dead fish kissing,
  2. holy crap I tripped and fell on your face type kissing (the absolute worst ever)
  3. pursed lip kissing,
  4. only hugging when given the setting and atmosphere it's most natural to kiss,
  5. having grown adult characters behave too prudishly about sex & intimacy -- like it's an absolute alien concept,
  6. not having the characters act like 'normal' lovers -- take the opportunity to kiss whenever they're alone, kiss each other goodbye or upon meeting (esp when they've been separated for months or years and are finally back together and all they do is ... HUG??? wtf ...).
You can be naive and inexperienced in sex, without having modern characters behave like they're living in the Silla dynasty. In It Started with a Kiss / They Kiss Again, the female lead was a virgin & as naive as they come, but she knew what sex was and didn't balk at the idea of having sex with the male lead.

I know it's all a cultural thing. What's shown on TV is heavily censored, esp in South Korea (same goes for Taiwan, Japan & China, but to a slightly lesser degree I'v observed). I get it, but I still don't like it. I avoid dramas where intimacy & sex are treated like a big pink elephant that no one wants to acknowledge.

So now that I've explained what 'good chemistry' is to me, here are a few dramas where I thought the (sexual) dynamic & interactions between the leads were somewhat 'natural' or as close to 'natural' as you can get in Asian dramas:

  • Lovers and City Hall (both of which are written by the same screenwriter as Descendants of the Sun).
  • Too Late To Say I Love You (most believable chemistry I've ever seen in an Asian drama)
  • Aishiteru to Itekkure
  • It's Okay, That's Love and The World that They Live In (both written by same screenwriter)
  • Loving, Never Forgetting
  • The Princess' Man (not quite, but to an extent)
It's Okay, That's Love
Reply1997
I Need Romance 3
In Time With You
It's Okay That's Love
Oh My Venus
Fated to Love You
Boss & Me
Rooftop Prince
You Who Came from the Stars
Love Me if You Dare
The Greatest Love
Oh My Ghost
Cheese in the Trap
Heart to Heart
She Was Pretty
PrettyCarEye has pretty much taken all that i wanted to say and typed it out for me lol. Just wanted to add though that casting has a lot to do with it.
i.e. in Battle of Changsha, When they first meet in the drama she is 16 and he must have been in his late 20's. The story line is that to get away from the war, women were married off to men so they could leave the war so not caring about what age the person was. To cast for this there was a 10 year age gap so intimacy is greatly reduced due to the RL age. There was a few kiss scenes but mostly just hugging and imagination. Didn't take any shine out of the story though.

That being said I wanted to add that Bromance is very believable. The actual story line though is crap but it's a better watch than you think.
Also greatest love, and lovers.
Based on chemistry, not story :

Reply 1997-1994
Master's Sun
Fated to Love You
Big
Pinocchio
Kill me Heal me
Healer
My Princess
Bride of the Century