I haven't seen everything ever but my top five so far:
1) Healer - for Ji Chang Wook and the fact it had just the right level of action and romance and a thrilling plot.
2) Kill Me, Heal Me - I'm a sucker for anything with Ji Sung and this is no exception. I love the romance, and the different personalities. It was just so funny!
3) City Hunter - I went into this thinking I wouldnt enjoy it so much, but this was FANTASTIC. Lee Min Ho, fast paced action, mystery and deception, what more could you want...
4) You're Beautiful - Okay, so it's a bit cheesy and a bit cliche but I LOVE the actors in this one. :)
5) I Hear Your Voice - Blew me away... loved the concept, and Lee Jong Suk was awesome. The plot blew me away though, I was hooked on every episode!
Honestly, I think Cheese in The Trap is gonna be one of my fave dramas at the end of it all... so far I'm addicted!