If you have never played The Secret Of Monkey Island, don't forget to play also Monkey Island 2. Those are ones of the best adventure games ever created. And if you're interested in Harvey's New Eyes, it is better to buy it from GOG instead of Steam, because for the same price you get the first game too (The Breakout) which is, by the way, a really good game.

Counter Strike Global Offensive is a great time waster if you ask me

Try metro 2033

Another good choice. But you really have to love very deep strategy games to go for that one.

Deus ex 1 is known to have slight issues on x64, when you get it from steam. I would recommend you the GOG version instead. I've read you can't buy it directly but it's not that hard to get a copy if you actually want to, just look around the forum.

This said, I still think DX1 is the best game on steam, simply because for me it's the best game ever. I'm happy you asked for subjective favorites, that way I need not hold back. If you have a problem with old games, then you probably shouldn't have asked for the favorites of a community in a site mostly dedicated to classic gaming. If you asked for modern favorites instead, I could have chosen something else.