Depends on whether I enjoyed the anime. Well, obviously I wouldn't keep watching the anime if I didn't like it, but there are different levels of how much I enjoyed animes/mangas. And with the ones I really liked; I definitely feel a bit sad that it's over, and want more. I think some notable examples would be the Dragonball series, Fullmetal Alchemist, Gantz (the manga; the anime wasn't really that great) and so on. I'd love to put Yu Yu Hakusho on here too, but unfortunately I felt like it died a bit after the Dark Tournament saga. I honestly don't really have any animes where I felt happy that they ended, since most have either left me wanting more, or are still on-going. Well, perhaps Sword Art Online. It was great in the beginning, but started going downhill after ALFheim became a thing, and it just kinda needed to end before it really went bad :(
Not much else to it. Guess I'm just really greedy when it comes to the good plots ;D