I really love this topic. I even made an article based on this idea...it was more general though. Anyway here is my list:D

1. Bi Dam & Seon Duk (Queen Seon Duk)
2. Chen Ling & Qi Luo (MARS)
3. Cheol Soo & Byung Hee (What's Up Fox?)
4. Hiroto & Nao (Tatta Hitotsu no Koi)
5. Kang Ma Roo & Seo Eun Gi (Innocent Man)
6. Kyohei & Sunako (YNSH)
7. Zhi Shu & Xiang Qin (ISWAK and TKA)
8. Ojiro & Kairi (Love Shuffle)
9. Seung Yoo & Se Ryung (The Princess' Man)
10. Lee Gun & Mi Young (Fated to Love You)

It's hard  to pick only 10 couples  ><

So, here is my list in no specific order :

1) Lan Ling Wang x Yang Xue Wu in Lan Ling Wang
2) Hiroki x Futaba in  Soredemo Ikite Yuku 
3)Yuan Xiang Qin and jIang Zhi Shu in ISWAK and TKA 
4) Takeru x Ruka in Last Friends 
5) Nakaji x Haru in Sunao Ni Narenakute 
6) Chen Ling x Qi Luo in Mars
7) Feng Teng x Xue Shan Shan in Boss & Me
8) Liu Shang Feng x Lin Jia En in In a Good Way
9) Zhong Tian Qi x Xia Zhi Xing in My Lucky Star
10) Kim Seung Yu x Lee Se Ryung in The Princess' Man

I'm a huge fan of the Eita/Ueno Juri couple , don't know why ...

Here's my list in no specific order (subject to change in future):

Kim Joo Won & Gil Ra Im  (Secret Garden) https://www.youtube.com/watch?v=0lvghDEieC8
Choi Han Kyul & Go Eun Chan (Coffee Prince)
Jo Min Hyuk & Kang Yoo Jung (Secret)  https://www.youtube.com/watch?v=--amAD-LR0I
Park Sun Woo & Joo Min Young (Nine Times Time Travel)
Makino Tsukushi & Domyouji Tsukasa (Hana Yori Dango)
Feng Teng & Shan Shan (Boss & Me) https://www.youtube.com/watch?v=jZ9ggUrbgbg
Do Min Joon & Cheon Song Yi (You From Another Star)
Cha Dong Joo & Bong Woo Ri (Can You Hear My Heart)  https://www.youtube.com/watch?v=RsI8SCYFqCE
Hiroto & Nao (Tatta Hitotsu no Koi) https://www.youtube.com/watch?v=NMUDxvwxuwE
Narumi & Suga (Piece) https://www.youtube.com/watch?v=Gf1p_gJRCBc
Ren Guang Xi & Liang Mu Cheng (Autumn's Concerto)

Hmmm, never really thought about this... It's a bit hard because there are a lot of dramas I adore in which I love one of the characters much more than the other, so my favorite dramas don't always correspond to my favorite couples... In fact some dramas I rated horribly have my favorite couples :/
Let's see...

1. Kang Tae Joo & Han Eun Soo (Que Sera, Sera)
2. Jang Hye Sung & Park Soo Ha (I Hear Your Voice)
3. Feng Teng & Shan Shan (Boss & Me)
4. Chen Momo & Zhuang Jun Nan (ToGetHer)
5. Kim Mi Young & Lee Gun (Fated To Love You)
6. Domyoji Tsukasa & Makino Tsukushi (Hana Yori Dango)
7.  Kang Woo & Han Myung Wol (Myung Wol The Spy)
8. No Eun Seol & Cha Ji Heon (Protect The Boss)
9. Toyama Sakura & Saeki Hiroto (Last Cinderella)
10. Do Min Joon & Chun Song Yi (You Who Came From The Stars)
1. Shan Shan + Feng Teng (Boss & Me)
2. Ren Guang Xi + Mu Chen (Autumn's Concerto)
3. Ling + Qi Luo (MARS) 
4. Young Shin + Jung Hoo (Healer) 
5. Ma Roo + Eun Gi (Innocent Man)
6. Makoto + Tohru (Rich Man, Poor Woman)
7. Boong Do + Hee Jin (Queen In Hyun's Man) 
8. Kotoko + Naoki ( Itazura na Kiss - 2013)
9. Kyohei + Sunako (Yamato Nadeshiko Shichi Henge)
10. So Yi Jung + Chu Ga Eul (Boys Over Flowers) - I swear I loved them more than the main couple!

I swear love a lot more couples but these are the ones that came to mind and I do love them!!! >U<)~
Right now I guess It would be impossible for me to rate again my favourite couples. Time to say watched too many dramas ^^