Please don't use word "popular". After all Naruto, Bleach and Pokemon are "popular" *gets shivers on the back of the spine*
My favourite series are usually 24 - 30 eps, like Cowboy Bebop, Scrapped Princess, Last exile, Trigun, Wolf's Rain, Noir, Samurai Champloo, Ergo Proxy, Trinity Blood, Gantz, Claymore and a whole other bunch that I can't think of now.
Sure there are some exceptions like Great Teacher Onizuka and Fullmetal Alchemist, but most as a rule it is under 30 eps, usually 24-26 and sometimes a few special episodes.
Also if Death Note would be only 25 ep long, it would be perfect. I regret watching episodes after L was no longer there.