When they aired Ren & Stimpy in the early 90s (when I was about 6, 7 years old), I didn't get it, and, no sir, I didn't like it.
Then, they re-aired the show on Nickelodeon in 2003, in a late-night block alongside SpongeBob Squarepants, Rocko's Modern Life, and Angry Beavers. These four have become my favourite Nicktoons, and I was absolutely mesmerized by Ren & Stimpy when I watched it as a teenager. It simply appealed to me and my state of mind a lot more at that time. Nowadays, however, I wouldn't watch Ren & Stimpy as gladly as I would 7-5 years ago.
Kablam!, yes, I remember that show. It was quite creative in exploring different animation techniques, though most of it was 2D, and some was stop-motion. Action League Now was hilarious.
Must also agree with everyone who said Hey Arnold. I found the characters in it were not as obnoxious as some other 'school-based' shows, and it really had its own unique poetic feel, with some dry humour and also slapstick thrown in. I thought Doug was a bit more tame and conservative than Hey Arnold, though I quite enjoyed Doug, too, until it went Disney.
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clarity of thought before rashness of action