It's very difficult to compare cartoons between eras. Duck Tales, for example, I hold in very high esteem, but it was formulaic, shallow and some of the animation keys were not so great (always a problem with Disney TV shows). Also, they kind of jumped the shark with Gizmo Duck.
SpongeBob was great... once. Talented voice actors, a good variety of gags, and random, unpredictable situational comedy. Also, terrific sense of comedic timing and whimsy.
Once
There's only so much you can do within the confines of a children's show, and it has lost a lot of its luster in the last few seasons. Some people are claiming a monumental decline...personally, I think it's just because the joke has run its course and the audience want something different.
Clever to make us vote on the first five seasons, however... I'd give it a subzero.. Not the biggest fan of the show (I like Avatar better) Few cartoon characters reach iconic status quite so quickly. And love him or hate him, SpongeBob is truly iconic.