Well for me, the following gave me that "WOW" feeling for a long time:
One of the first has got to be Quake. This was very impressive to me and started my whole PC gameing experience.
Next came SimTower.. ahh the gameplay was what's so great. I got Tower Rating also! Took many trys..but I eventually made the ultimate tower.
Age Of Empires 1 (very addictive at first for a long time..2 wasnt as addictive..later I even tried Rise Of Nations.. some said it was good.. i say it's crap.. gameplay is too busy.)
Stronghold and Stronghold Crusader (both very amazing.. they blow AOE away!)
Rollercoaster Tycoon (ahhh.. now here was something to play!)
The Sims (CRACK COCAINe.. i regret the hours wasted tho..:-( )
Grand Theft Auto 3 (Play and Beat #3, then get vice city! AMAZING games... I Looooove this new genre they busted out with.)
Halo (BUY IT!!.. you wont regret the purchase..online play is the best!!!)
Right now I'm trying Tiger Woods PGA 2004 and Doom 3 (good graphics..lacks horror..might be the save system that makes it too easy)...oh yea, dont forget some HALO from time to time..damn it's such a time waster...but soo fun... drool