(Just a note - all the songs I'm including are 00s songs - nothing really old I guess, since I only know a select few from 80s and early 90s, but I've listed them as old as most were released 3 years+ ago!)
First off is the song that got me into kpop in the first place. It's ironic that it's actually a pretty lame song overall, but it sparked my curiousity.
1. Super Junior - Knockout
I found the whole "13 members in one group" concept ridiculous at the time. But then my interest was peaked by their individual and all very humorous personalities. They were my first favourite group and are still my favourite overall group to date.
The first kpop song that got me hooked on the choreography was Wonder Girls' Tell Me. It's the only song where I've learnt the entire choreography from start to finish.
Other old kpop songs I like:
SHINee's Replay. This is my ultimate favourite Korean song of all time. It reminds me of my youth just listening to it! It came out before I turned 20 so I always associate this song with the end of my teen years. Classic song and as with most SM tracks, good choreography too.
DBSK Wrong Number (disliked Yoochun's "cracked bowl" hairdo though, lol)
Jewelry's One More Time: