Although Taeyang possesses one of those unique square shaped faces, he is always seen in his "official" haircut: the extremely short onion haircut.
"Onion" haircut as in: shaving 95% of your head, leaving the front 5% of the top forehead with a traditional "fohawk".
However, one aspect Taeyang's haircut is good for is having the impression of clean and lean.
Now, back to the actual reviewing of the onion haircut:
In the current Kpop industry, most guys often wear the popular medium haircut with bangs. This makes Taeyang's haircut fairly unique and classy. However.... It's called sloppy as soon as the Kpop idol wears the same haircut for over 5 years. I mean, it will be nice to have Taeyang having a Kim Bum hairstyle sometime.. Not only does Taeyang's haircut look like it was quickly shaved in 5 minutes, without any effort, but it also reflects on his style as well. It gives an impression that the onion hair was never polished up at all.
To be frank, Taeyang's haircut:
Lacks style (considering the amount of time he has had this haircut)
This is going towards a 6/10 for style
10/10 for maintainability (You don't even need to maintain it!)
5/10 for the short length (there's not much thickness and length to his hair, it's heading towards bald)
To conclude, Taeyang's hairstyle is pretty nice overall, as long as you don't put on this hairstyle for years in a row.