Samurai Jack, TMNT (the first three or four seasons or so), Venture Bros (maybe not exactly underrated, but it's so good I think it deserves even more praise), and the awesomeness that is Johnny Bravo.
Oh, and Animaniacs. Yeah, it's not really underrated, but I'd say it's easily one of the top 3 cartoon series' ever made (The other two probably being Samurai Jack and a tie between South Park and Venture Bros).
Oh, and Animaniacs. Yeah, it's not really underrated, but I'd say it's easily one of the top 3 cartoon series' ever made (The other two probably being Samurai Jack and a tie between South Park and Venture Bros).