It depends on how you define best seed. I like to have a mushroom fields island, all biomes within 3,000 blocks, all structures in 3,000 blocks, and some cool terrain underground as a bonus. Other people like certain biomes or want a large biome of a certain type at spawn. Others want cool structure generation and like seeds with multiple structures colliding or close. Others are only interested in the most beautiful terrain generation in which to build. Other still are only interested in seeds perfect for speed running. To each their own!
I have found this java seed 523356022724029296 with a mushroom fields island within 500 blocks, mansion and withch hut within 2,000 blocks, all structures within 3,000 blocks, all biomes within 3,000 blocks. I have looked less into the Nether and End, but they should be good.
The seed I am currently playing on is 1920597807115725671 and it has some cool features. However, even though all biomes are roughly 1,500 blocks of 0 0 and spawn, the closest woodland mansion is 6,600 blocks out. I also wonder in cramming all biomes within 1,300 to 1,500 blocks leads to less desirable samples of each terrain a missing structures.
Does anyone know the best survival seed for the newest version (1.21) Like in terms of generation, loot, biomes and such?