As many have said, it is quite difficult to narrow it down to 10 games, because there have been so many great games over the last few decades! I think a lot of my picks will be due to good memories/nostalgia, but that's usually how these lists go I guess.
The Witcher 3 (PS4)
This game just blew me away in terms of scale, graphics, storytelling, characters... It is an amazing game that just keeps offering something new with every road you follow, every corner you turn. The views in this were breathtaking as well. I spent so many hours on this game, finishing very sidequest, finding all the Gwent cards, going to every question mark for the treasure there... Loved it.
The Legend of Zelda: Majora's Mask (N64)
Now for the longest time this was my top game ever (often switching places with OoT, as I rate them similarly). MM was a great sequel to OoT, using the same characters but putting them in this dark and twisted alternate dimension. It had such a great atmosphere, and that looming moon overhead really did freak me out. I think they really nailed it in this game in terms of character development and storylines. The Anju & Kafei quest was just great, getting to swim through the water as a Zora, or roll around the field as a Goron... Or defeating enemies as Fierce Deity Link, it all felt fun and new and exciting.
The Legend of Zelda: Ocarina of Time (N64)
This is the game that got me into the Zelda series and properly into videogames altogether I think. I had been playing games before that of course, but this one felt like something completely different to the other games that were out there. I had also played some of Link's Awakening before this but didn't realise it was part of a game series (no internet and no video game magazines yet at that point). I spent ages at the start just going through the Kokiri village and the Lost Woods, thinking that this game felt huge... and then I ended up in Hyrule field and was blown away by the size of it all haha. It's nothing compared to games of today, but at the time it felt huge. As with many, this game defined gaming for me, it had everything: good graphics, great storyline, memorable characters, fun gameplay, an amazing soundtrack.
So this game was my first experience with the Souls type of games... and it was a very frustrating experience at the start! I had seen @Fierce_LiNk play through this and struggle at the start so thought I knew what I was getting into, but I still wasn't prepared for it! I almost gave up at the start, but persevered and in the end this game is one of the few games I've got a Platinum trophy for. The monsters and bosses in this are scary and deadly, the whole Victorian gothic style looks amazing too. It's a creepy game, but I loved all of it.
Rollercoaster Tycoon 2 (PC)
This game might be a weird one to put on a top 10 list, but I absolutely love(d) Rollercoaster Tycoon. I spent so many hours building my theme parks and designing rides, and this second game in the series just perfected what was already there in the first one. I recently installed it again on my laptop and I had just as much fun going through it now as I had back then.
Overwatch has been one of my first online shooter experiences (apart from Star Wars Battlefront). I've sunk in over 400 hours on the game (I have no life) and am still not tired of it. They keep adding maps and heroes to the game which keeps it interesting. It's just a very fun game and I love playing it. I imagine I'll still be playing it for a while to come too.
Age of Empires 3 (PC)
God I spent so many hours on this game too! I played it loads as a student, often playing well into the night. The skirmish games were just fun, and I enjoyed creating a base and slowly building up my army. It just felt very satisfying to beat your enemy with your massive army haha.
Fire Emblem: The Sacred Stones (GBA)
This game and the previous GBA one got me into the series, which I absolutely love. I've not really ventured into the latest ones due to a lack of time (and fading interest in handheld gaming), but I loved all the other games I played. I seem to have a thing for tactical games, and this one was different in that your characters can actually die. It brings a new level of play to the game, making you more cautious about your next move.
Super Smash Bros. Melee
I think that this one and Brawl are my favourites in the series. The pacing and characters in it are perfect, and I had a lot of fun playing it. I also got to play it with other people at the time (rare for me as my family is not into gaming), so it meant I got to enjoy it more than for example the N64 one. Still a fun fighting game and a must have for any Nintendo fan.
Advance Wars: Dual Strike (DS)
Again a tactical game, I really enjoy them. The Advance Wars series made war seem fun heh. It's sort of like an advanced chess game, where you have to plan out the next few moves in your head, and try to think what the enemy might do. I really loved it and wish they would make a new game in the series, but it seems they are only focusing on Fire Emblem these days.
Animal Crossing: Wild World, Mario Galaxy, Golden Sun, Tetris, Horizon, The Last Guardian, Journey, Pokemon Red/Blue/Yellow, Kirby's Dreamland...