Best games on Steam?

This is my first post, so apologies if I've done something terribly wrong, but I just played Portal 2 on PS3 and got a steam account. Now I want to get into pc gaming and I'm looking for the best games on there, either old or new. Also, should I wait for the steam summer sale? Or is there something that I should just splurge on right now? tl;dr Looking for the best steam games EVER