Bride, I'd have to say it's Sim City 3000. I've been playing that since it came out - 10 years ago? or so? I like it because it is different every time you play it, whether you use the pre-existing city plans or start from scratch and make your own terrain. Plus the cheats are good!
I much prefer puzzle-type games - not much for the shoot-em-ups. Big Fish has a pretty good assortment of those.
http://www.bigfishgames.com/
Got hooked for a while on the hidden-object games but have sworn off them as they are all basically alike, tho with slightly different twists.
I played Peggle when it came out - didn't want to pay what Popcap wanted to download it!