Popular is a pretty relative term. Currently, series such as Naruto and Bleach have ongoing popularity. New iterations of the Mobile Suit Gundam series, such as Gundam 00 are regularly highly popular when they premiere. Meanwhile, series such as Neon Genesis Evangelion remain popular, though it has been over a decade since it originally aired. If you want to go by number of episodes, Doraemon is probably your winner, as it clocks in at nearly 2,000 episodes and continues to grow.