Luckily, this is one of the few cases where democracy actually works. With over 50% of the vote, TAS is the obvious winner, with diminishing returns for the later series, as one would expect. TAS was a cultural juggernaut, with the ratings, toys, Pizza Hut deals, comics, behind the scenes book and movie spin offs to prove it. Even with greater animation(and budgets), the later series just can't hold a candle. Was TAS perfect? Absolutely not, but what it lacked in production value it more than made up in heart.
For those who say it hasn't aged well; look at the mature themes it explored. The government spying(Mutant Registration Program), the flying death machines(Sentinels), the angry radicalized mobs(Friends Of Humanity), the weaponized/politicized virus( Apocalypse's Legacy virus), these are all things you could read about in the news to this very day, and this is from a Fox Kid's show 25 years ago.
Season 1 is near perfect(they didn't know they'd get more than one season, so they went hard), the Phoenix and Dark Phoenix sagas are better and more accurate than the movies, Sanctuary 1/2 has Magneto throw down with the UN and explores the politics of a savoir figure being betrayed by his own power hungry supporter, episodes like Slave Island and Storm Front 1/2 directly confront slavery and empire, Beyond Good and Evil 4 parter is a huge mega crossover, Apocalypse and Beast spout philosophical gems every other line, Charles and Magneto's rivalry and friendship threads the entire series, the main X-Men are friends/family/and a badass team, and non-members Cable, Bishop, Archangel, Iceman, Colossus and Nightcrawler get meaty focused episodes, a fully formed Alpha Flight and X-Factor even get some shine.