I think the most addictive game I ever played was Dominions II a fantasy setting TBS game. It had appallingly simple graphics but a tremendous strategic depth, carrying the rock paper scissors model and the resource management element to the max.
One of my favourite RTS games of all time must be Total Annihilations. There is just so much fun seeing all those little machines making a bee line for the enemy. Game balance was not optimal though, the air component was far too strong in vanilla TA
These days there is TA:supreme commander but much more fun is the freeware TA:spring where you can actually get into a first person perspective.
For a first person shooter I always enjoyed Duke Nukem most. At times it was even scary when you could just here there had to be one of those pig cops around the corner but could not see him.
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It is time
To break the chains of life
If you follow you will see
What beyond reality