Mineclone2 New Ores and Deepslate
NO11

Adds 30 new deepslate blocks!

Caverealms Lite
Shara

A mod for Minetest to add underground realms.

Baked Clay
TenPlus1

Adds the ability to bake clay into blocks and colour them with dye.

Cloud Items
Panquesito7

Powerful cloud tools.

Multiple Biome Generator
chaosomnium

230 Pseudorandomly Generated Biomes, Trees, Fruits, Ores, Tools, and Liquids

Melterns
IcyDiamond

Tinkers Construct in Minetest!

Quartz
Evergreen

Adds quartz ore and some decorative quartz blocks

SkyRealm for NodeCore
Warr1024

SkyBlocks inside vanilla NodeCore

Base Modpack
karlexceed

This combines several other mods that I consider a basic minimum starting point for worlds.

Darkage - the original plus improvements
addi

DarkAge adds several new nodes and crafts to create a pre industrial landscape.

Alien Tools
chaosomnium

Alien Tools Mod for Minetest - adds Alien Tool, Shield, biome, etc

Australia
demon_boy

Adds a wide variety of Australia-themed biomes and content

NodeCore Nature: Revised
Warr1024

Updated, fixed, enhanced edition of Winter94's NodeCore Nature

New Horizons
Hi_World

Adds 4 new biomes and TONS of new blocks!

Obdy
Sires

Adds some pretty OP tools :)

Tac Nayn
GreenXenith

Adds Nyan Cat's evil twin, Tac Nayn (and a gray-scale rainbow).

Oxidizing Copper
StarNinjas

This mod aims to make copper/bronze more realistic by making them slowly oxidize into a bluish/green block when exposed to water.