Is deus ex 1 worth buying? It feels very.....old.
Well Deus Ex does feel rather old, if you prefer newer, then Deus Ex: Human Revolution is also very good and should be on sale for 75% sometime this weekend. It's currenty half off. It's a really good game.
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.