Tough choices. I watched all three religiously as a child. On one hand we have Ren and Stimpy which had a distinct visual style, crass humor unlike it's time, and two characters who have a severe dsyfunctional partnership with one another. Armed with crazy situtations (and the occasion Powder Toast Man episodes), it kept me entertained for a good while.
With Doug, I had a character I still find myself able to relate. While I'm not mild-mannered in any sense as he is, his active imagination and the ability to put it on pen and paper is what drew me to him (pun probably intended). Armed with a quirky, uniquely drawn cast, the show had a lighthearted sense of drama and comedy that worked perfectly. It was like a breather show you could watch after something intense.
And Rugrats. I could care less for any episodes that came after the first movie, the seasons before were where it is. In an overall long run, I think this show would be the one I'd choose over the other two. Other then the vastly interesting concept (who'd have thunk babies would make good television material?), it's the character's interactions with one another as well as their crazy and oftentimes surreal adventures that intrigued me (Tommy's solo journeys that's usually wordless were some of the best and a good look at how babies might perceieve the world).
But mostly, it's Angelica. She's an obnoxious little devil in plain sight and her personality is one of the prime reasons I can't stand a lot of children, but being the first manipulative jerk I've come across, I enjoyed all the little mind games she played on the babies; I enjoyed her reeking in her victory, I enjoy how she can twirl circles around her parents, and yes, I even enjoy her comeuppance (because she deserves 'em).
So, there ya go. My two cents.