Sonic the Hedgehog Series - First games I ever played. Inspired me enough to look into becoming a games developer later on. Turns out I didn't like actually making games as much as I did playing them. Didn't venture any further with that

Doom 1/2 - Ah, the number of times I snuck into my Dad's office and stole his Doom floppy discs.
RuneScape - As much as I hate admitting it, without RuneScape i'd have never gotten into the online multiplayer games at all. Sure, I could play Halo online, but that didn't really compare to the soul-sucking, addictive time-sink that an MMO can provide.
WoW - Possibly the most natural progression was between RuneScape and WoW. Though I don't particularly play either much any more, WoW got me thinking much deeper about game mechanics and how story is very important to keep a player playing the game.
Pokemon - No other game in the history of the world ever has taken so much of my free time, I've racked up well over 4000 hours (I just checked my game saves XD) across the series and I still play it

There are more, probably, but I can't think of them right now
