Probably in order:
1) Scatman's World by Scatman John
- I've always been a huge fan of Scatman. I personally feel Scatman's World was his best.
2) Gangnam Style by PSY
- Really infectious. And the lyrics make sense.
3) Daydream Syndrome by Fujiwara Marina
- Personal favorite, no real reason.
4) Connect by ClariS
- This song is partly here because of it's attribution to Madoka, and partly because it is a good song.
5) Bang! by After School
- I never could understand why I liked this song.
6) Infinite World by SOUND HOLIC
- The first SOUND HOLIC song I listened to. This is why I love SOUND HOLIC.
7) Tsuki no Murakamo Hana ni Kaze by senya
- The supposed second opening to Memories of Phantasm. I don't care about the series too much, but I love the song.
8) Venus by Shinhwa
- Incredibly catchy, and the music video. Still can't keep mishearing 'Venus' in my head.
9) Moskau by Dschinghis Khan
- Listened to it a while ago, got stuck in my head, and I liked it.
10) DotA by Basshunter
- Up here, but at the bottom. Catchy.
And other notable stuff:
- Earthquake Super Shock by SOUND HOLIC
- Volume Up by 4minute
- Sakura Kimi ni Emu by yozuca*
- CLIMAX by Mami Kawada
- Jet by f(x)