Killing Floor is so addictive. It uses an old unreal engine, but the gameplay is great, especially if you play on 30-40 player servers, it's like a MMOFPS. The players are mostly mature "old-school" players, not those whiny jerks in Left 4 Dead 2 servers. Oh yes Plants vs Zombie ^ is indeed very addictive too. I bought it on steam and finished it in 2 days. Never thought something that looked like a flash game would be THAT addictive.
The best game ever is sims!!! I have played it since the very first one came out and now have all the sims3 games and always follow when the new one is to come out so I can pre oder it!!
My favourite games are strategy games like long time ago Dune2, Age of Empires, Cossacks. The best is to play them with friends on a LAN.