Because it didn't had the PS2's casual appeal AND break-down rate. Don't forget the humongous library.

 Ironic that you like the PS2 which had tons of Shovelware but don't like the Wii

Gamecube here, reliable hardware, great games like Zelda WW and Melee (that literally lasted all the years of the GC), and had the most fun with friends with it.

Tie with PS2 and Gamecube.

Oh by the way... I thought GC had the best rated games last gen (Metroid Prime and RE4)... did I miss something?

 opinion =/= fact

GameCube>>>>>>>>>>>>>>>PS2 IMO. 

Did you forget where you are, and the majority of the preferences here?

PS2 hands down, no question. 

It is an impossible question to answer. It comes down to what each persons definition of BEST is.

Best graphics/ best game play/ number of sales???

IMO it has to be the PS2. the others may have had a greater potential given the hardware/ exclusive IP's - but overall nothing came close.

Think for a few minutes about all of the games. Think of the graphical difference between early PS2 games to God of War II.

every genre was flooded. If sales and consumer dollars are any indication of what the "best system" was. Then it was a resounding PS2

Best in absolute terms, or personal favorite?

Super Smash Bros. Melee is my favorite console game of all time, and I don't care about GTA or GT at all, so the Gamecube is probably my favorite. However, the PS2 definitely has a stronger library and wider appeal alltogether, even thought it has less games that I like.

For me it was the Gamecube. Even though it was the only console I had last generation, I think that if I had the other ones I would still prefer the Gamecube for having more games that fit my tastes.