There's 3 Minecraft worlds I have that I'd say are tied for the best.
1: On PS4 I made a survival world themed after the solar system. It looked very good until I switched it to survival and my progress messed up the planets.
2: Also on PS4 I made a world so I could have a unique way of presenting a project for class (I did it this way as a joke). After the project I started building random things throughout the world and some looked pretty cool.
3: On mobile I had my favorite world I've ever made. It was pretty cool because there was a desert and I built some palm trees so it looked like an oasis resort and right by it was a gladiator arena. I also made an ice cave, a nice small house, and a village that I was working on. The highlight of the world was this giant castle I made called Castle Obama. It had a nice design, in and out, and took me about a couple weeks to work on (including time spent not playing). Sadly about a month ago this world disappeared for some reason and all my progress was lost. I have another world where I'm making a new Castle Obama but I know it won't be as good as the first.