I think I liked the whole ragnarok online soundtrack, at least the stuff of the first 2 years. If I listen to the soundtrack I still know most of the places the sounds belongs too, especially prontera theme. Spent much of my playtime hanging around there, just sitting down and chatting with ingame friends as nice break between the xp grind sessions.

Still regret a little bit that I took a break to play WoW, because WoW was offering so much stuff that made the daily MMORPG life easier that I missed much of that in ragnarok online when I went back.
Although I had a great time in WoW, and probably was the best time of WoW. Especially the raid encounters and how the druid class played are the things I miss most in ddo. DDO only has a few raids who can compete with my memories of the raid encounters in WoW. VoN 6 is one of them, best VoN6 period was when groups needed at least 2-3 rounds of eggs befor velah was down and people still had to hide behind the rock because of the dragon breath.
But I am getting off topic.

Command & Conquer 1 also had some nice music, just don't remember the names of the ones I really liked. ^^
Portal 1 & 2 ending songs. (still alive, want you gone, turret song)
https://www.youtube.com/watch?v=B9RIHOnGGsg