Season 2 is definitely worth finishing.
Season 3 has more of the same classic humor, with a lot of new introductions:>Ice King's mysterious past.>The Universe beyond Ooo.>Susan Strong, another human.>Flame Princess, who is introduced in the finale.
Season 4 focuses heavily on Finn's burgeoning relationship with Flame Princess, but a majority of the episodes still evoke the classic feeling from seasons 1 & 2. Don't bother with any episodes that have Tree Trunks in it.
Season 5 is the longest and most diverse season, going from intense universe ending story threads, to the up until now unexplored back story of Finn, to more relationship drama - probably the worst in the series, episodes focused on minor characters - some more important than others, and still some great standalone adventures - such as Dungeon Train. I liked it, but you may not, who knows.
Season 6 is the first season to be composed of more weird episodes than fun ones. There is some crossover between the two types - such as "Food Chain" - but most are more garbage like the continuing plot of Tree Trunks and Mr. Pig, speaking of which you should've actually watched the wedding episode from season 5 because it introduces the King of OOO, the best character in the series after Magic Man.
Getting back to the topic of season 6, I feel like this is the best season finale the show has, even if a majority of the plot remains unanswered, at least it feels satisfying.
Season 7 is mostly weird stuff and the Stakes miniseries. The weird stuff isn't quite garbage yet, and I liked Stakes even if everyone on here hated it.
Season 8 is the beginning of the end. This season , like the previous one, is only 26 episodes, like the first couple seasons of the show. But, since both this season and the last have a miniseries in them they feel much shorter. cont.