I don't have that many clear milestones I think. I think they would be:
1. Pokemon - I don't remember how it all came to pass, but I didn't have my very own (legit) Pokemon game before I owned Crystal. Awesome series, that I spent a lot of quality time with.
2. Star Wars Episode I Racer - The first home console game that could just keep me hooked. I have clocked a lot of hours in this game, and it was time well spent! This was probably the defining game for the 5th generation of homeconsoles for me, along with Pokemon Stadium.
3. Spiderman 2 - The most awesome Spiderman game to date, and my brothers and I played the hell out of it. For me, this was one of the two defining games of the 6th generation, the other being:
4. SSX Tricky - This game was awesome beyond belief. If I had to choose one game I would love to see make a comeback on the HD consoles, it would be this. A proper SSX Tricky reboot.
This is where things started to change, and I became more aware as a gamer. When I was younger, I could just play a few awesome games over and over again. But not any more.
5. Guild Wars - The only MMO-type game to ever have me hooked. This game defined my PC gaming years more than any other game, and taught me the meaning of skill and brains in gaming.
6. Warcraft III: The Frozen Throne - I tried to play through the SP, but no dice. I'm not an RTS player. But the custom maps were the star of almost every LAN party I have gone to, and they are still awesome today. LAN parties would be empty for me if not for Warcraft 3.
7. Metal Gear Solid 4: Guns of the Patriots - This game became the reason for why I owned a PS3 once I got it. This is an amazing game, and is still probably my all time favorite. I had never played a MGS game before, but this one still came through, in grand style.
8. LittleBigPlanet - I played through this with my friends, and never before had local coop been so fun. So many hours of pure fun have been had with this game, just playing local multiplayer. It still amazes me.
9. Flower - I had considered games a form of art for a long time, but before this game came along, I had no shining example. But Flower is just that. A game that conveys feelings like no game before it, and is art in every sense of the word.
There, those are the games that stick out in my mind. 
