Categorically

World gen has been overhauled with Geolosys, spreading large clusters of ore throughout the world, rendering strip mining essentially useless. 

BetterEnd overhauls The End, adding new biomes, mobs, blocks, and more, making the end somewhere that you actually want to be. 

One of the gnarliest Nether mods to date, Infernal Expansion makes the Nether actually feel like the hellscape it tries to be.

The Underdark is the only new dimension added to the pack so far, as it has plenty of depth and uniqueness to feel like somewhere worth visiting.

Wesley's Roguelike Dungeons are added to give you something to mix things up with occasionally. You'll never run out of dungeon to delve with this.

YUNG's mods (Better Caves, Better Mineshafts, Better Dungeons, Bridges, and Extras) add plenty of exciting new structures to come across, as well as enhancing the ones that are there.

Tetra is the tool base for the pack, adding highly customizable tools and weapons for various materials, even those added by other mods.

Seeds adds waste, waste management, recycling, watering cans, mud golems, bad grass, pollution, ways to clean the atmosphere, sparkling water, and more. Very extensive and very cool.

Small Ships adds...small ships. Greatly improved over vanilla boats, small ships have controllable sails, inventories, and look much cooler. Use these to navigate your oceans without being bored to death.

Ex Nihilo: Sequentia is in the pack in order to offset any frustration around not finding Geolosys plutons for the ore or gem you need. It also enables the pack to be fully played in a Skyblock. (Included, WIP)

YUNG's Better Portals drastically changes the way you enter the Nether. You'll first need to find a Rift near the bottom of the overworld to enter, and then you'll need to find a Monolith in the Nether in order to leave. Once found, however, you can pick up the Rift in a bucket, and collect the Monolith to move it somewhere more convenient. 

Alex's Mobs is the most extensive wild mob mod out right now, and it adds mobs that will really make you feel like the world is alive, all while fitting into the vanilla feel.

Ice and Fire: Dragons brings that epic fantasy world to your door, adding fantasy creatures, new structures, and a genuine big boss to the overworld that feel satisfying to surpass.

Improved Genetics is a bit of a gamble, with less than 10k downloads at the moment. However, the potential for this mod seems great, adding fun new variants of vanilla mobs, and giving them a use as well.

Sapience makes the Piglins, these humanoid creatures in the Nether, actually make sense. They are now intelligent, and will make the Nether feel less like a wild animal free for all.

Plant-Based Magic-Tech mod, Botania makes your connection with Minecraft's nature be rewarded with efficient processing, automation, and more.

Occultism takes another route, relying on rituals and demons to take advantage of the world around you.

Project: E (and addons) aims to reduce the grind of the pack in later stages, if you so choose. (WIP, EMC values are subject to change, addition, or removal)

Reliquary Reincarnations adds magical items and blocks that are difficult to acquire but make life much easier in various ways.

Create is an aesthetic approach to automation, that both elevates allows you to process and automate many things, as well as looks amazing doing so.

Immersive engineering is a retro-futuristic machine mod, with an emphasis on realism, multi-blocks, and more. It's also integrated with Create for more cool combos.

Iron Furnaces is just a quick and simple tiered furnace mod, allowing you to keep your furnaces upgraded as your progress through the pack.

Tom's Simple Storage is a very simplified version of a storage network, mainly focused around linking your storage via cables to a central interface.

Iron Chests is a quick and simple tiered chest mod, allowing for the extension of your chests storage capabilities in order to save on space, as well as looking better.

Metal Barrels is a complement to both Iron Chests and Iron Furnaces, once again allowing for tiered upgrades of your storage via barrels.

Farmer's Delight is a thorough but not convoluted farming and food overhaul. It incorporates wild plants, new crops, new foods, and new methods of cooking.

Nether's Delight is an addon to Farmer's Delight, extending the functions of Farmer's Delight into the Nether, allowing for increased immersion and survivability.

FTB Quests is added with the intention to add various tutorial quests to help introduce you to some of the lesser known mods. This is still WIP, however.

FTB Chunks adds a simple mini-map with waypoints, as well as basic chunk loading.

Quark, and Quark Oddities, have been added in order to really spice up the little corners of the game you may not have realized needed some spice.