These are my favorites, I'm sure at least some of these have to be on yours as well.
My top 10 of all time (quick list off the top of my head. one title per series)
1. Starcraft (PC 1998)
The best RTS ever made. Simple as that. Starcraft is not a game, it is a way of life. The fact this is being written on TeamLiquid.net right now is a testament to the legacy that Starcraft built.
2. The Legend of Zelda: A Link to the Past (SNES 1991)
My pick as the greatest title in probably the best video game franchise of all time. Link to the Past created the formula that all Zelda titles still follow. It introduced things like multiple worlds, the Master Sword, the Hookshot, the spin attack and the heart piece fragments. When it was ported to the Gameboy: Advance more than 10 years after it originally came up it was still amazing.
3. World of Warcraft (PC 2004)
There will probably never be a video game title that ever sucks me in the way WoW did. Like Starcraft WoW is much more than just a mere video game. Even though it's only ranked third on my list it is the only game that I can say with a straight face that I have been playing consistently for 10 years and I have no plans on stopping (on a break until WoD though :D)
4. Super Smash Brothers: Melee (Gamecube 2001)
My all time favorite fighting game. Super Smash Brothers: Melee is actually two games in one, it's both one of the greatest 4 player party games of all time and one of the finest competitive 1 on 1 fighting games ever created. It greatly improved on the original which was itself a groundbreaking and incredible game. While Brawl managed to successfully expand things like the player roster and the single player campaign beautifully it was a step backwards in the competitive area and for this reason Melee is still considered by many, myself included, to be the greatest title of the franchise.
5. Mario Kart 64 (N64 1997)
Let me ask you a question. Do you know ANYONE that doesn't like Mario Kart? Nuff said.
6. Poke'mon Red/Blue (Gameboy 1996)
Poke'mon did for me in third grade what WoW did for me in High School. It brought a video game into the mainstream culture in a massive way. Arguably one of the greatest RPG titles of all time and easily the most successful handheld gaming franchise ever, Poke'mon isn't just a game it's a cultural icon.
7. The Elder Scrolls 3: Morrowind (PC 2002)
Of all of the of the fantastic Elder Scrolls games I still love Morrowind the best. Even before WoW sucked me in to a world filled with other people, Morrowind managed to immerse me in a way that I never knew was possible. The first time I ever played Morrowind was nothing short of magic, it was as though I was living inside of a beautiful movie.
As much as I love Skyrim and Oblivion and many of the new features they added, there's nothing that can capture the original magic that came from playing Morrowind and in a lot of ways things like fast travel and quest markers detract a bit of it.
The one thing Morrowind doesn't have that its descendents had was the unbelievable modding community (It existed but it wasn't nearly as extensive) that keeps Skyrim trucking along today.
8. Donkey Kong Country 2: Diddy's Kong Quest (SNES 1995)
My favorite childhood game. Also my vote for all time greatest video game soundtrack
9. Rome: Total War (PC 2004)
While Medieval Total War 2 is probably my favorite overall of the Total War titles, it was really Rome that laid the groundwork for everything the later titles would build off of. Unbelievably deep strategy with an impressive amount of detail paid to historical accuracy, Rome: Total War is truly an epic experience for anyone that enjoys strategy or history.
10. Street Fighter 2: Turbo (Arcade 1991)
My first fighting game and still an absolute classic. My love of fighting games and in truth competition via video games in general all came from Street Fighter.