The first one that comes to mind is the game that I have the most memories of, my absolute favorite game of all time, Pokemon Sapphire.
I've been playing Pokemon games since I was about 3, but I never really got into them. I mean, I always loved playing them, but I never had a game of my own that I absolutely loved and treasured. Red was given to me by my brother when he got Yellow, and it was shared with him most of the time, so I couldn't really call it mine. It was mine, but since he had given it to me and I was too young to really understand it, since I was about 4 or 5 at the time, I never really felt the same connection to it that I eventually would have with Sapphire.
By the time Gold and Silver came out, my brother stopped loving the games, even though he still played his Yellow on occasion, so my sister and I each got one. I got Silver and she got Gold. We played each others game fairly often, though, so Silver still wasn't really my game. My sister eventually grew out of Pokemon as well, so Silver is the first one that I consider to have been my game. However, by that time, I had played fairly far into it already with my sister, so I didn't get to experience a whole new game by myself, and I felt the same way with Crystal when I got it for Christmas that year.
I got really into Silver when I was in about second grade, and was friends with a lot of people who also got into Pokemon from their siblings, and all of us were excited for Ruby and Sapphire. (One of the kids actually made bookmarks and stuff and sold them to other people for money. He's actually always found ways to make money off of what was popular at the time, now that I think about it.) A little bit after Ruby and Sapphire came out, since we all were smart enough to think of the fact that they'd be worth less in a few months, everybody in the group of friends got them, whether they bought them or got them for their birthdays, myself included.
When I first turned on my GBA and began playing Sapphire at Chili's on my 8th birthday that year, I was amazed. The graphics were so amazing, the Pokemon were so cool! (Torchic is still one of my favorites.) Things as simple as the String Shot animation astounded me. Abilities and natures were such great ideas, giving every Pokemon that extra little something that made them unique. Team Aqua was so cool. They were pirates! Pirates! The music was such better quality, every single track gave a route, cave, or town that feeling that all gamers get sometimes, where they feel like they're in the game. I absolutely loved this game. It was perfect. For the first time in my life, I had a brand new experience in a brand new game all by myself, and, to this day, I've never felt anything greater.
I continued playing that game day and night, since my birthday was in the summer. I got together with the group, especially one who I'm still friends with today, and we traded and battled and just generally had a good time playing the games we loved.
During that summer and the next year, I actually made a lot of friends since Pokemon was huge at our school and I was able to help them all with anything they asked for, if it was that one rare Pokemon that just wouldn't show up, how to evolve and get the cool Pokemon you've always wanted, or just some tips on how to beat a Gym Leader. All of them were temporary friends, ones who I don't see anymore, but I still had a great time with them, playing games and chatting on my pretty bad ProBoards forums. I do miss some of them, since they were great friends who I just kind of drifted apart from because they moved or went to another school, but it was fun while it lasted.
The game itself just has so many memories for me. The smell of tar makes me think of training on Route 121, my favorite Route, on the long car ride to the cabin we used to go to in Minnesota. Mattresses with floral patterns of any kind make the memories flood into my head of training and training on my bed in the cabin to finally take down the Elite Four, and of the glorious moment when I finally defeated Steven's Armaldo. Hospitals remind me of a day one summer, when Volturnus the Kyogre became my first level 100 as I was visting my cousin in the hospital. There are so many more, but if I kept going,I'd name them all, and this post would become far too long.
All of those things helped to make playing Sapphire one of the greatest experiences I've ever had, and it is the one Pokemon game I own which I've never restarted and never will. I've wasted a good 20 full days on that game, and I'm not going to let all that time be for nothing. I'll always value Sapphire more than any other game. I'm sure that the game itself, the great experiences that came from it, and my team will all hold special places in my heart forever.