Hrm... It's kinda difficult to choose... Zelda's have always been good, Mega Man (for most part) stayed fairly consistent with the appeal they had from the beginning and adding a story to each sequel, the Mario's will always be classic no matter how they put them, Final Fantasy always have the strategy and involving plots, Metroid's got the rapid action and cool 'additions' (along with being one of the few series to consistently have a female as the main character), Castlevania hasn't fully faded away from the mainstream... but it has improved over the years, Sonic has the speed appeal as always, the Wild ARMs series is one of the most unique RPG sets I've ever seen in having (for most of them) Wild West themes, and the Xenosaga/Xenogears set are worthwhile for the time involved in making and playing them.
So as you can see... I can't make up my mind. ^^;; ((All game names have been bolded.))