I would say Yu Yu Hakusho because when I was getting tired of Dragonball Z, this came along and just BAM! Smacked me in the face with fresh and interesting characters, fights that didn't feel drawn out and padded, even if they were, and made the ruler of hell suck a pacifier.
But Kou already mentioned most of that.
Sooo I shall mention what is actually my favorite manga series and a pretty amazing anime as well, Rurouni Kenshin! It was the first show I saw that had a rather meek main character, at least outwardly, so I was pretty amazed when I see the stark differences within Kenshin whenever he had to face someone from his past. There was also something about the action that I just loved, no energy shooting or anything, but really fun and fast sword play with insane strategies involved. (Makoto Shishio's defeat was so crazy and left me on the edge of my seat) I think I would call it my favorite anime of the 90s if it had ended better. I didn't mind the fillers in the end, but I suppose it's what you could call a prelude to the Naruto style filler in that it just went on and on, only here the series just ends. I would have loved to see the entire Enshi arc animated and still would, though I think seeing it animated in the style of the 90s would have been interesting because even in the manga, it felt like there was a visual difference. Was not a fan of the OVAs because they just took the animation and story in such a hopeless direction.
I'm also curious as to why some people don't consider things from the 90s to be old school. They're over 20 years old now. There really is no official marker to justify what is and is not an old school anime, but as has been mentioned, an entire new form of animation was made the industry standard since then. There have been incredible changes since the 90s and I think 20 years is definitely enough time to say a show has become old school. (you never really heard anyone in the 90s saying stuff from the 70s wasn't old school)
