Longest series I've ever seen is DBZ. Far as I know I've seen all 291 episodes, including the ones skipped during Gohan's training with Piccolo as Toonami did air them once.
Second would be Sailor Moon, though given I like the DiC dub for the early episodes, there are a small handful of those I've never actually seen. So I've only seen maybe 195 of those. Not including some rare episodes like that one for an obscure VHS player phone toy.
Otherwise I think most of the anime I've seen that's long caps at like 70 episodes at the most. I know I've seen a lot of shows like InuYasha, Naruto, and Bleach, but I ended up stopping all of them for various reasons. Long before I likely got to 100 eps in.
As for a long show I watched all of in just one year. Probably Monster when Sci Fi did ani-monday's. Assuming it was all in one year and not something like it began in October and went till April or something. I can't remember.