Longest and most favourite games for me are AOE 2 and Half Life (with the two mods).
I dont know how many times over for almost 7 years i have finished the games in different difficulty levels.
I would say Half Life 2 could not do what Half Life could do in 1999.
Another absolute favorite is San Andreas. Even if the game story line is finished, nothing stops you from roaming about awesome designed cities or wreaking havoc on the busy streets, speeding away in a CRV and fooling around the army region in a stolen air craft
.