For the past year or two...
King of the Hill
But of all time?!...
RUGRATS!!!
Yes, Rugrats! I absolutely loved this show. I watched it every night with my sister, and sometimes my dad, religiously through most of my elementary school years. The show was absolutely fantastic, and i don't believe any Nickelodeon show has been as great about moral values, family ties, and just plain kids having fun as this show. The downfall came when that little neusence Dill caused Tommy to stop adventuring and thus making him be the softy baby sitter... what a jerk.
On a side note, i also enjoyed
Doug
All That!
The Amanda Bynes Show
Kablam!
Wishbone
Cat Dog
...and The Fairly Odd Parents (the last great show by Nickelodeon before I couldn't stand the crap they were dishing out any longer... and I was too old to watch the shows anyway)
Nickelodeon where did these classics go!?!?! And what the hell is the Naked Brothers Band?!?!