Miyazaki's best work isn't Spirited Away. It isn't even a Ghibli film.

I agree with you. Future Boy Conan is his masterpiece. I absolutely adore that show. Even though certain aspects would be improved upon in his later works, such as the animation, music score and even the creativity, but in terms of story-telling, emotion, plot and especially character, he nails those aspects the most out of anything he has done. The show is an emotional experience.

Each individual will and should have their own perception of what each artist's greatest masterpiece is. For me it's Kiki's Delivery Service, for others it may be something different. As for Future Boy Conan, I haven't watched it yet but I plan to when given the time. I'm not sure movies should be compared to a tv series though, many different factors to consider.

"Unless you're an alien, time traveler, or esper, your opinion doesn't matter."