The Animated Series pretty much defined the X-Men for me. The Sentinels could easily have suffered from the
Conservation of Ninjutsu (where uniform enemies that come in swarms tend to be incompetent and useless), but each one was BAD-ASS and every time they showed up, you knew that things wouldn't go well for the X-Men. As an example, in the very first episode 'the plan goes to hell' when the Sentinels show up and the X-Men are barely able to escape... but Morph and Beast aren't so lucky.
All of the characters get plenty of development and even better, you see how they interact with each other (Cyclops and Wolverine are at odds over Jean and Storm often has to intervene, Gambit and Rogue have definite chemistry but Rogue's power makes things difficult and often funny, many of them coddle Jubilee, Wolverine and Morph are best friends gone wrong, Beast... well Beast's in prison).
All the best storylines get covered, too... the Registration Act, Sabretooth and Wolverine's war, the Savage Land, Jean becomes Phoenix, Phoenix becomes Dark Phoenix, Bishop and Cable's time-travelling, Apocalypse and Genosha, the Weapon X program, Omega Red awakes, Juggernaut's vendetta, the Shadow King escapes the Astral Plane, and all of this without even including any of Magneto's Brotherhood of Mutants storylines.
The show was possibly the greatest cartoon of all time.