HALF-LIFE 1 and 2 and thats it, no other games are ever needed after that.
You finish the game and you have a massive line of mods to play after,
in theory the little 50 or 60 euros yuo've just spend on them could easily give a playing experience worth 600 euro but for free.
E.g I finished HL2 back in november but i've still been playing it although i havent touched the plot since with mods like GMOD, Empires,Battlegrounds,Zombie Panic,
Brainbread,Sven coop,art of acension,weekday warrior (although its loaded with trojans),(LOL)troy divine playground,combine destiny,The Hidden,Natural selection,Sourceforts,Half life 2 substance and thats just half way down my installed list (actually a little less down).
Don't forget steams built in voice communication.
I rest my Case.
P.s they also provide the modding software free of charge with blistering fast download servers.