I'm not going to repeat a lot of good ones I saw on here already, but the ones I feel like had the most impact on me/like a REAL lot were FMA both seasons, Eureka Seven, FLCL, Zoids, Digimon.
I honestly don't remember why I dropped it after starting it, but in the beginning it didn't really hold my interest. But then I started reading the manga, and after I found out what happened in that I started watching the series again and it's one of my favorites now.
The only filler arc that I thought was good was the Zanpaktou arc. I hated the rest of them. But they were necessary to put space between manga releases and episode development.
But if Isshin lost his shinigami powers through using the Final Getsuga Tensho, then how can he still use his shinigami powers if it supposedly takes it away after using it?