I think "most nostalgic" is ultimately a subjective concept, but as for me, the most nostalgic cartoons were the ones featured on late 90s, early 2000s Cartoon Network. That was easily my favorite channel growing up and I watched it religiously. Out of all the cartoons, the most nostalgic for me was the Powerpuff Girls. Everything from the characters to the music are legendary. I used to have everything Powerpuff Girls; dolls, shirts, VHS tapes, and even CD soundtracks. My favorite one featured a song by a forgotten band called the Apples in Stereo. I think the song was called "Signal in the Sky", and they would play the music video during commercial breaks sometimes. I also adored Buttercup. She was my first idol. I always thought she was so cool and tough, as I was more like Bubbles growing up and hated myself for being so sensitive like her all the time. Also, Buttercup wore green, and that was and still is my favorite color.
Some honorable mentions would be Dexter's Laboratory, Johnny Bravo, and by far the biggest honorable mention is Courage the Cowardly Dog. That one was my second favorite after PPG.