Top Party Game for Casual Gamers:Pac-Man Vs.

Super Smash Brothers Melee can be picked up by anyone and be able to have fun, it's a simple, fun, game. Also, Four Swords Adventure and Mario Kart: Double Dash!! are honorable mentions on good multi-player Gamecube exclusives.

I recommend:

Zelda: Four swords adventure
Final Fantasy Crystal Chronicle
Cel Damage
and Mario Party 5,6, or 7 for anybody who had never play to a Mario Party game.

My friends and I are constantly playing Mario party 4 and 7 but Super Smash brothers Melee is one of my all time fave games and its a really fun multiplayer game.

Two multiplayer games for the Gamecube which are incredible are Super Smash Bros. Melee and Wario Ware Mega Party Games. Both I strongly recommend.

The best games to play with your significant other:

-Coop:

-Baldur's Gate, Dark Alliance: It was a very simple game. She loved collecting stuff and leveling up her character. If she died, there was usually a place close enough to revive her, so she was always playing with me.
-X-men: legends I and II: It is very similar to Baldur's Gate, but the leveling up is a bit more involved. You do not get too attached to your avatar since you are changing characters all the time, and there are some special missions that require you to play as single player in Legends I!. When one of us died, we still had two more characters to use... however if the three die, reviving them is not as easy as in Baldur's Gate. Then again, this is simpler in part II thanks to the portals.
-Lego Star Wars: Very fun but very short as well.
-Lord of the Rings: The Return of the King. Reasonably fun.

I had Limited success with the TMNT 2 game. I only bought it to unlock the arcade game and relieve old memories.
Pikmin2 is still on trail. Unfortunately you can't play the mission in cooperative mode, and thus you only have some minor quests where you can play cooperatively.

I had bad luck with
*Timesplitters 2: She doesn't like FPS
*Star Wars: Rebel Strike: Too difficult. She keeps dying and we cannot get past the fourth mission.

-Versus:

-Mario Kart: Double Dash: you can play all missions and unlock all bonuses while playing in two different screens. This game is so fun that my objective sometimes was not to win, but make sure she was the winner: by selecting the turtles, and trying to get or slow down everyone else... really fun. It is versus mode but somehow it feels sometimes like cooperative... even with split screens. (sharing a car together is not much fun becuase the person in the back gets bored).
-Sonic Riders: Not as fun as Mario Kart, but more challenging.

I had Bad luck with:
-Kirby Air Ride (for very small kids perhaps?),
-F-Zero (too difficult and not so fun with several players)
-Any fighting game like SoulCalibur, or even Smash Bros. Melee. To win you need to know combos (we are bad at memorizing them), or the other option is button mashing, and thus there is no strategy or talent to win.

--Party Games:

-Pac-Man Vs. The ultimate party game. We played it all the time whenever we have visitors. It is worth buying the gameboy just for this game. Any visitor can play this: Casual gamer or hardcore!
-Karaoke: Not our favorite, but when we have a party people tend to like it.
-Mario Party 7: When you have 8 people it is the best game to play. But we warned, you need to understand first the games so you can explain them to your party! Plus you need to play in the 8-player party mode, not the board game mode! The board game mode is boring.
-FIFA World Cup 2006! This is the best soccer game for cooperative play, and 2v2 matches. When you make a pass, you make it to your teammate, unlike old fifa versions where you passed onto yourself (last fifa game I played was fifa 2002).
-Tetris: It gets boring really quick, but when we have visitors is another quick game to play. This is a mixed bag nevertheless.


I had bad luck with:
-Monkey Ball: ok. maybe the one where monkeys punch each other with gloves is fun. But one out of eight party games is a low ratio.
-Mario Party in board game mode. None of my friends had the patience to roll dice and figure out how to win.
-Bomberman Generations: this is fun the first time you play it and for 30 mins at most.