X-Men (1992)
I realize it has been 21 years since this debuted, but it is close enough. Probably considered the best Marvel series, in my opinion second best comic book series to Batman, it received many great reviews, and it is currently the longest running Marvel series ever with 76 episodes. The show follows the X-Men which are pretty much identical to the second X-Men comic that came out in the early 1990s. The shows follows Cyclops, Wolverine, Jean Grey, Jubilee, Beast, Storm, Rogue, Gambit, and Professor Xavier. The show has had many, many more X-Men characters show up, just not as main team members. The show covers have famous storylines, even though loosely followed. The Dark Pheonix, Days of Future Past, the Phalanx Covenant and more. I highly recommend this to anyone who enjoys anything comic related.
Fantastic Four (1994)
This show came out a couple years after X-Men, and I'm sure was created due to the success X-Men had. This show as part of a "Marvel Hour". Each hour had one episode of Fantastic Four and one of Iron Man. In no way does this series have the quality of X-Men. It really is for Fantastic Four fans, I don't see non-Fantastic Four fans getting much out of it. To be fair though, the second season is extremely better in every aspect than the first. It is much more complex and less silly. The show lasted two seasons and twenty-six episodes. Both seasons were on Netflix recently, but apparently have been taken off since. The entire series can be had on DVD for around $10 on Amazon though.
Iron Man (1994)
Iron Man is probably worse than Fantastic Four. Everyone seems to be an Iron Man fan after the films, but I still don't know if it is worth watching. As with Fantastic Four, the second season is completely different and actually pretty decent, but damn that first season sucks. The Mandarin has created a group of super-villains to take down Iron Man. The combat this, Iron Man creates his own team: Scarlet Witch, Hawkeye, War Machine, Century, and Spider Woman. Most of these episodes are completely original, especially the first season. Very little comic inspiration here.
Spider-Man (1994)
This is probably the second best Marvel animated series. I might like Avengers more, but it would be a close third place. This show is somewhat similar to the previous Marvel shows, but it seems to have a bit higher production budget. The show went over my head a little as a kid. The storyline can be really complex, I still don't get the last two arcs of the series. It also deals with some mature themes while still managing to be kid friendly. Now go back and watch it, Spider-Man hardly ever punches anyone. I swear he did more often, but I guess that was just implied. Anyway, it is the second longest Marvel series, and I recommend it to any Spider-Man or cartoon fan.