So so so many.
PS:
- Neopets: the Darkest Faerie - This one is for PS2. I freaking love this game. I've played it more than I care to admit, and I never get bored of it.
- Spyro - this goes since PS1. The first one is my all-time favorite, but they're all great.
- Crash Bandicoot - also since the PS1. The spin-offs are awesome too, especially the racing one where you can "mix" with other runners.
- GTA SA - I loved killing people. My mom didn't know about the violence, so the secret made the game even better lol
- Kingdom Hearts - omg this game is just incredible! I was addicted to it, and I could easily get addicted if I started playing again. I can't wait for KH3 to come out
Nintendo:
- Zelda - any game of this franchise, really.
- Pokémon - all of them, even the spin-offs. I don't care about that "original 101" bullshit, the more, the merrier!
- Mario - most of them as well. They're great to play when bored
- Elite Beat Agents (DS) - this. Game. Is. Fucking. Awesome. It's one of those music tapping games, but this is not like the other ones, this game has great music and gaaaah it's just so fun!!! I used to play on my DS lite with an R4, but after getting a 3DS I couldn't play it anymore. A couple years later I found a copy in an online store and bought it, now I can play it any time
XBOX:
- Just Dance - I have all of them lol. It has a lot of girly dances, but I still play them. It's great to play with friends and family, but can also be played by yourself. They get better every year, with better dances and better costumes too.
- Far Cry 3/4 - I haven't played 1 and 2, but 3 and 4 are amazing. Played for hours.
- Dead Rising 3 - I tried DR4, but it didn't have sandbox mode, which is what I love about DR3. I don't like the pressure of saving other people and being careful not to turn into a zombie. I just want to craft weird weapons and kill zombies!
- GTA V - basically for the same reason I love GTA SA, killing people. The graphics are much better too, of course. I really liked the story of the game.
PC:
- The Sims - I've played since TS1 (I was really young but that didn't stop me from making them "woohoo" [thanks Trich]). Sometimes I get bored for a month or so then get back to playing it.
- Minecraft - I like playing single players maps and some servers have pretty cool games, but the best thing about Minecraft was when we played for the first time on a server. I miss those days.
I'm sure there are a lot more games I love, but these are the ones I can think of.
Edited to add some other games.