I think this poll is more "which Nintendo system came out when you were between the ages of 6 and 10?"
If you ran this same poll 10 years ago the N64 would likely have been rated highest, and the SNES highest if run 10 years before that. 10 years from now maybe people will be agree it was the Wii or Switch that was the greatest Nintendo system ever! Wii U might never quite make the cut, sadly...
As much as I love it, I have to note that the GameCube was a really low point for Nintendo, and in many ways for Nintendo fans too. The games came out slowly, 3rd party support was rapidly waning, and the system was in 3rd place far behind the PS2 for sales. Of course, there's some real gems on this platform that were well-loved and undoubtedly defined the childhoods of millions of players around the world, but purely from a, say, dispassionate viewpoint, GameCube sold poorly and had a fairly small game library. Afterall, the blue ocean strategy of the Wii was a direct response to the GameCube's bitter defeat when trying to compete purely on power and graphics.
I was born in the late 1980s, so of course my pick is SNES! It was the big new Nintendo console of the day during that impressionable young age, so of course my dearest childhood gaming memories are linked to it. Had a massive library of games that really pushed the envelope, too. Super Mario World, Link to the Past, Super Mario Kart, Yoshi's Island, Mario RPG... what treasures!