That's a good question, Best slap tone ever?
I'm not sure if I've heard a slap tone that I've been particularly transfixed with, but I will suggest Larry Graham.
I dig Larry Graham, and his thicker sound is more attractive to me, as opposed to the thinner sound favoured by modern exponents.
I like Louis Johnson's playing, but for me that's more about his playing than his tone, but he still has a thicker sound than modern slappers.
I like Marcus Miller's sound, but if I listen to an entire album, I get tired of the sound. I think his tone is best on M2, but I'm also inclined to think that particular album is his best work so that could make me biased.
I like Victor Wooten but it has never been his tone that attracts me, first his gymnastics and facility got my attention, now it's his phrasing that I like, I actually enjoy his playing more when he is not slapping.
While I may not be decided on who I think has the best slap tone, I do know that there are tone attributes that I do not like, and I hear those characteristics in famous and non famous players. I find that when the tone is too bright and too thin, it just gets annoying and I can understand why this technique is often disliked.
Come on folks, slap all you want but don't abandon the base function of the bass, make sure you tone is supporting the music you play, rather than highlighting your special double thumb, double pluck Wooten/Riendeau hybrid lick.