GTA:SA and Age of Empires II.

After a while i get bored of any game but the game which made me addicted for the longest time has to be Final Fantasy X. I played that for two years.

Demon's Souls and Mario Kart: Double Dash.

Super Metroid. This has got to be my all time favorite game, I beat it like 1-3 times every year (and I rarely if ever replay games).

gears of war, mario kart ds, resident evil 4, and super mario world

Depends on what you mean by never get bored. If you mean what I can pick up and play and still enjoy, then I have a host of games. Ranging from Ocarina of Time, Castlevania Symphony of the Night, Muramasa: The Demon Blade, Metroid II, Smash Bros Brawl, Capcom vs SNK 2, Chrono Trigger, Dragon Quest VIII, Sim City 3000, etc.

However, as for games I play consistently that I never get bored of, I don't really have one of those. I can't just play a game non-stop forever.

Dota, Hot Shots Golf 5, Rock Band (1/2/Lego/Beatles/...), Warcraft 3, Starcraft, Anno 1404, Mad TV, Capitalization 2

Perfect Dark (and now the port will make it better :D), Starcraft, Mario Kart Wii (online) and yes, this will sound kinda lame but Tetris Attack/Pokemon Puzzle League (same thing, different characters).

I have been playing those year after year and idk when i'll get bored.

mario kart wii online
uno wiiware online matches

Pokemon Snap