Simple as gaben eating pie, just rate your 10 favorite currently owned steam games, things like software count as well. If you only have 10 games, then go ahead and lower it to Top 5, here's mine. Oh, and if you get a new favorite game, go ahead and edit your original post.
10. Sonic Adventure DX 9. Scribblenauts Unlimited 8. Bit.Trip Runner 7. Cave Story + 6. Source Filmmaker 5. Batman Arkham Asylum 4. Sonic Generations 3. Portal 2 2. Garry's Mod 1. Team Fortress 2