i like marceline and her songs but she's way too old for him. Bubblegum is too old but marceline is way too old..so in that case i'd go with flame princess though i really want bubblegum to be 13 again so finn can be with her
Even though you might want Finn and Marceline to be together. It's not likely to happen in the show. She even straight up told him that she wasn't interest in him in that way. It was in that episode that involved a couples only drive-in theater. And it was obvious that he didn't see her in that way either. They just see each other as friends. But in the land of fandom, anything is possible (not in the show though).
As for who he should be with I would say adventure but if I had to chose an actual character, I would say Flame Princess since she's just about the same age as Finn.