Credits and sources at the end of the video. Thank You...
These are another my top 15 (not best 15) my personal taste. I don't have the right to judge which one is the best from all of his songs and I am sure everyone has a different preference too.

30. Shake It Up    • Seo In Guk (서인국) "SHAKE IT UP" Music ...  
29. Calling You    • Seo In Guk (서인국) - Calling You (부른다)  
28. Our Love Like This    • 정은지&서인국 - 우리 사랑 이대로 (응답하라 1997 Offici...  \
27. Brand New Day (Jellyfish Ent)
26. With Laughter or With Tears    • 서인국(Seo In Guk) - 웃다 울다 (with laughte...  
25. Open Your Eyes
24. Fate/Like A Fool
23. Bad
22. Mo, Aenakute.    • ソ・イングク「もう、会えなくて。」MV  
21. Bebe    • 서인국(SEO IN GUK) - 'BeBe' Official M/V  
20. Flower    • [내일 그대와 OST Part 1] 서인국 (Seo In Guk) ...  
19. Time Machine
18. Light of My Life
17. Love Again
16. I Can't Live Because of You https://www.youtube.com/watch/gG1N9...
Bonus:
Shout Out To The World