Probably a tie between Plains and Mountains.

I remember a server that I played on where I found a small plains surrounded by mountains with a bit of a hilly mountain jutting into said plains. I decided to make a tunnel system under some rock plains that opened out into some ramparts outside winding up said hill to the mini mountain where I built a fort. Pretty fun build and I was kinda neat having a rather long tunnel system with 2 portals inside and a ventilated stone fireplace in one corner.

I did have to use the hoe and sometimes rock flagstones on the outside as a way to cover up open sky inbetween using boulders for a more natural look.