GameCube.
The first-party titles I've played are really enjoyable, and there's plenty I haven't played. I like some of the third-party games as well.
But really most of the Nintendo systems I've played are relatively comparable in quality, so this was tough.
3DS deserves more love. It's my favorite pure Nintendo handheld (never used Switch Lite and Switch is a hybrid).
Lifetime Sales Predictions
Switch: 161 million (was 73 million, then 96 million, then 113 million, then 125 million, then 144 million, then 151 million, then 156 million)
PS5: 115 million (was 105 million) Xbox Series S/X: 48 million (was 60 million, then 67 million, then 57 million)
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima