Usually my level of enjoyment of a drama is highly dependent on its quality (especially the cast), so it's pretty easy to stay objective on whether it deserves a 10 or not. Although there are exceptions. My 10s:
Hana Yori Dango - exhibit A of an exception. So many flaws in this drama (especially the first season), but I loved it so much, I can't bring myself to give it anything below a 10. It was also my first Asian drama, so its got that special place. MatsuJun played out the character exactly as I'd pictured when reading the manga, and better. Which never happens with manga/book-to-films.
49 Days
My Name is Kim Sam-Soon
Secret Garden - Probably my all-time favorite drama.
City Hunter - I usually wait until I've finished a drama to rate it, but this is so good and so beautifully done that I was compelled to go ahead and give it a 10 before the episodes had even reached the double digits.
Greatest Love may receive a 10 as well, but we'll see. And I'm already loving Myung Wol the Spy, but we'll see. One that probably deserves a 10 but can't bring myself to give is Coffee House. I even dropped it at about 14 and skipped around from there. It may have just been me, but although the Lee Jin Soo character was genius and perfectly done by Kang Ji Hwan, I could not bring myself to like him with either female and couldn't stomach watching the rest (the Eun Young character is fantastic on paper, but the actress really rubbed me the wrong way). Maybe one day I'll go back and watch the rest of the episodes just because I adored Jin Soo so much. It's worth giving it a shot just to watch him.