This is really tough to narrow down to two...
Meadowland is really REALLY cute with the waterfalls, so I'm very partial to that one currently.
Riverlands being the OG and very water-based will always have a place in my heart, and I think as a relic of a legacy map, maybe it would have waterfalls too if it were introduced alongside 1.6.
Forest always has that mystique and intrigue for me, and it had been the one I really want to play on next... something about above-ground backgrounds bring out the creative juices in me, and I felt there are lots of decoration opportunities using those overhead foresty leaves as concealing components...
Hilltop also feels very nice with complex geographic features like tiny mountain ranges and rivers. However restrictive the terrain may be, I feel like this would be another one I would have fun decorating on... and having unique features like the Geode node is always a bonus.
.. and maybe because Beach farm was ALWAYS my "perfectionist" farm, I was getting kind of sick of it, but it is honestly a very good farm too, the crates are especially interesting and since I'm not very much the farmer-type, the rough terrain is not a minus for me!