For 1.19 and after:

minecraft:normal - A flat world with no features, can be modified with generator-settings.
minecraft:flat - A flat world with no features, can be modified with generator-settings.
minecraft:large_biomes - Same as default but all biomes are larger.
minecraft:amplified - Same as default but world-generation height limit is increased.
minecraft:single_biome_surface - A buffet world which the entire overworld consists of one biome, can be modified with generator-settings.

For 1.18 and before:

default - Standard world with hills, valleys, water, etc.
flat - A flat world with no features, can be modified with generator-settings.
largeBiomes - Same as default but all biomes are larger.
amplified - Same as default but world-generation height limit is increased.

For 1.15 and before:

buffet - Same as default unless generator-settings is set.
default_1_1 - Same as default, but counted as a different world type.
customized - After 1.13, this value is no different than default, but in 1.12 and before, it could be used to create a completely custom world.

Minecraft MOTD Generator / Creator