Alright class, history lesson.
I saw in the comments that @ibMIMI wanted to talk more about older groups and guys, I'm here for you.
Here are some of my favorite tracks from before 2010!
TVXQ - Wrong Number (2009)
Alright, basically every song by TVXQ (when they were all 5 members) is a JAM. Do yourself a favor and fully immerse yourselves in the Cassiopeia world.
SS501 - Love Like This
Fun fact, the people who wrote this song also worked on Oh My Girl's Closer! I actually saw them perform this at the Korean Music Festival in 2009 in LA :D SUCH A JAM.
TVXQ - Mirotic (2009)
TVXQ again. This song is ICONIC and people still reference it all the time. The dance is soooooooo great.
BoA - Eat You Up (2009)
BoA is such a queen, she even went on tour with Britney Spears! Also check out Hurricane Venus which came out in 2011!
SHINee - Replay (2010)
GAH this is when I lost my MIND for Key. This is still one of my favorite SHINee songs ever and is so, so sooo good.
Super Junior - No Other (2010)
I have to admit, I love everything that SuJu put out pre-Mr.Simple. This is an example of their cuter, lighter stuff and its SO FLUFFY.
Epik High - One (2010)
Epik High was one of my first loves of Kpop and this song still hits me like a brick wall. It's stunning and PLEASE look up the lyric translation, its amazing.