I'd say Kino no Tabi. Well, that's the fate of an anime like that, I suppose. It does have a fanbase, but only consisting of those that actually reached a certain age so that they can enjoy it to its full extent. Under normal circumstances I'd probably have named Mushishi as well, but due to Zoku Sho airing this year, it's actually getting a bit more recognition as well. Still underrated in my opinion, though.