Ethereal NG is an absolutely classic mod, it's one of those things that you usually add to a minetest modded game right away because of just how simple it is and how much it adds.
For specifics, my favourite parts of Ethereal are probably the biomes. I love exploration, and it's always a treat to go from a plain ol' grassland to something exciting, like a crystal biome. I also love the fishing mechanics in this game. The new items such as food and tools (not to mention nodes like bamboo wood, walls and all that jazz) expand upon parts of Minetest Game that I found to be lacking. Overall, this mod takes what makes Minetest Game not very fun and adds something new to make it fun.