From the 90s I liked the X-Men cartoon intro theme. The theme from Tiny Toons Adventures was also pretty memorable and catchy.
Going back into previous decades, I'd have to mention Flintstones for a memorable theme. I didn't like it too much though. Another example could be the Scooby Doo theme.
Personally I always liked the intro music that came along the very old cartoons from the 40s or 50s like Tom & Jerry, Looney Tunes or Popeye.