Mine's a three way tie between anything Pokemon, Fallout New Vegas, and Runescape.  Some of the most addicting like Runescape, always updating and all that good stuff so always new stuff to do.

If I had to pick just one game that was the most addictive to me I would have to say shining in the darkness I think it was a wonderful game that was a tad before its time in game play and graphics
my second choice would have to be any of the Mario games I really enjoyed playing them

Super smash brothers melee and super smash brothers project M. The brother I have the closest relationship with is the one I constantly play melee with.

Beautiful Katamari is one of the most hilariously addictive games I've ever played.

I just want to roll up everything. Again. And again.

And again and again.

Grand Theft Auto IV is second to that, though.

I think overall the whole Pokemon franchise is pretty addicting. Once you pick up a game you don't want to put it done until you get your next badge and its a constant cycle of that. Like for me personally, my first Pokemon game was Pokemon Silver version for the Gameboy Color. Even though I started off playing Pokemon on the second generation, I did go back and play and finish both Pokemon Red and Pokemon Yellow. And that's been the case ever since, like I've played Pokemon Red, Yellow, Silver, Gold, Crystal, Sapphire Emerald, FireRed, Diamond, Platinum, SoulSilver, and finally White. Alongside of those I've also played Pokemon Stadium, Stadium 2, Puzzle League, Red Rescue Team, XD: Gale of Darkness, and Colosseum. Its just something about Pokemon games that keeps its fans coming back and they're really easy to start playing. And once someone starts playing any Pokemon game they'll want to play the rest.    

Minecraft. It's like a curse, once you start, you never stop.

final fantasy VII for the playstationX

Any Harvest Moon game, and Future Cop: L.A.P.D. for the PSX.
I listed two, because they are two different types of addicting. HM keeps me coming back, but Future cop kept me glued to the system for hours on end. I couldn't get enough of Precinct mode, though the Campaign mode left alot to be desired in my opinion.

POKEMON PUZZLE LEAGUE idk it's crazy addicting sometimes it's hard to sleep because it on my mind alot

Super hexagon. No questions asked.

For me, it's the original Pokémon. You know what they say, "Gotta catch 'em all!"

Simpleeeeeee cave story I LOVE THAT GAME TO DEATH And Pokémon! I beat the rematches with the e4 in fire red

For me, the most addicting game is Eternal League of Nefia, or Elona for short. It's an odd RPG with roguelike elements. The amount of things you can do in the game is crazy, and there are skills for almost every one of them. It doesn't take itself too seriously, so you can run into some really strange situations.

But Harvest Moon and Pokemon Mystery dungeon are pretty addicting too!

The most addicting game I have played was Pokémon Yellow Version .

Definitely Mario Kart 64. I want a Nintendo 64 so bad, just so I can play that game on it. I just went a bought a Game Cube for the sole purpose of being able to play Mario Kart Double Dash.

civs 1-3   (whattya mean its 4 am??)

It took some thought and I would have to say Cladun X2.  I just simply love dungeon crawlers. (especially ones with rangeons)