The Empire of the Sun has chosen you to be the first person to step foot in an alien dimension . . . well not quite the first we don't know what happened to the last guy . . . but never fear! We have confidence in you. Of course if you don't report back there will be another graduate to follow you . . .

Optimized for Large Settlements

AI Improvement (AI mobs won't waste system resources just looking around, generally use less memory on fluff)
Better FPS (Improves your FPS)
ChunkGen (Every time you load the game it will pre-generate a number of new chunks so you don't lag as much exploring)
FastCraft (Much better game performance)
FastLeafDecay (Leaves will quickly decay once they are no longer next to a log block rather than taking up system resources of time)
LightingEdit (removes lighting lag at night/underground)
Scott's Tweaks (Lowers the number of bats and squids that spawn, also adds in more clay to world gen as a side benefit)
Fog Nerf (Gets rid of the fog that you find near bedrock for better performance)
BurnCreeperBurn (Creepers burn in direct sunlight like other hostile mobs removing lag)
CreeperCollateral (Explosions leave behind all blocks so you can rebuild if you survive.)
Open Blocks (when you die your stuff is stored in a "grave" so items aren't scattered about which could cause lag. The grave may spawn skeletons at night though so beware.)

A Beautiful Over World to Explore

Realistic World Generation mod (Gigantic Trees, mountains, winding rivers, biomes merge into each other smoothly without straight line chunk boundaries.
ExtraBiomesXL (More biomes to explore, tons of flowers, new trees, etc)
Natura (Giant Redwoods and other trees , berry bushes for early food)
Pam's Harvestcraft (Tons of new foods you can scavenge during your hunting and gathering days. Includes fruit trees and wild crop "gardens", villages have more crops)
Growthcraft (Even more early food. More drinks including beer, wine, and mead brewing, villages can have bars and grow hops.)
Tinkers Construct (Metal Gravels, floating slime islands, ore-berry bushes underground)
Underground Biomes Construct (Tons of new types of rock most with their own types of cobble. Get vanilla stone by crafting four new cobble types together when/where needed)
Weather, Storms, and Tornados (Amazing skies and dynamic weather systems.)
Dramatic skies texture pack (A small pack that adds a beautiful sun and moon to go with our amazing particle based clouds and weather systems)
Ancient Warfare (away from spawn you may find better villages, bazaars, plus castles full of bandits, pirates, etc . . . )

A Better Nether

Better Sleeping (Provides a bed in the nether you can sleep in if you want to live there.)
ExtraBiomesXL (nether biomes)
Natura (Giant Mushrooms, imps, nether trees)
Pams Harvest the Nether (More trees, foodstuffs, etc . . .)
Tinker's Construct (Nether versions of Ore Berry Bushes, some unique ores spawn in the nether)
NetherOres (All the overworld ores spawn in the nether!)

Realistic Survival

Better Sleeping (You can sleep whenever your character is tired. Not sleeping for long periods results in a debuff and eventuall you will pass out on the ground. Watch your sleep meter!)
Hunger Overhaul & Spice of Life (Both of these have been heavilly tweaked in the configs. You need to eat different foods frequently or you will slowly get less nutrition from your food. You will continue healing even when not at full hunger, but as your hunger gets ever lower you will suffer debuffs as you begin to starve and eventually you will stop healing and start losing health down to 1/2 heart)
Cooking For Blockheads (This adds in cook books that show you all the foods you can make from items found in natura, harvestcraft, growthcraft, vanilla, etc . . . it also adds a multiblock kitchen that makes cooking easy so you can get massive buffs later on and hugely saturating foods, hugely hunger satisfying foods, even foods with potion effects!)
Zombie Awareness (Zombies will be more alert. When you are hurt they can smell the blood and track you by it. Mining and other loud activities attract their attention. More Zombies spawn in groups at night for random invasions. They will clump up together and wander the terrain. Sometimes faster zombies spawn.
Weather, Storms, and Tornadoes (Storms, lightining, hurricanes that can rip up dirt and trees; plus tools to deal with it all of course)

Unique Steampunk Tech Mods

Ancient Warfare (The heart of your settlement with peasants to do all sorts of tasks like farming, herding, and running a warehouse)
Railcraft (The heart of your transit system; build huge networks of trains)
Tinkers Construct (Better weapons and tools made with a modular system to do what YOU want them to. Smelting metals. Lots of fun.
Immersive Engineering (big multibock machines)
Steamcraft 2 (More big multiblock machines, also steam powered tools)
Magneticraft (Even more big multiblock machines to build in your town.

Better Building and Storage

Chisel (Chisel blocks into a wide variety of decorative shapes)
Carpenters blocks (Wooden frames that you can color with other blocks. Slopes, doors, beds, ramps, garage doors, more)
Bibliocraft (Clocks, weapon racks, armor stands, chairs, tables, crafting benches, more)
Better Storage (Bigger chests, cabinets, and more)
Jabba (barrels that can hold tons of stuff)
Malisis Doors (More doors and a tool to let you design your own)
Lights Plus (many decorative lights made from redstone)
Garden Stuff (Flow pots, cast iron rails, decorative hanging lanterns, candles, modular gardening system, more)
Harvestcraft & Growthcraft (also add candles and some other stuff)
More (railcraft adds lights, the steampunk mods add lanterns, rusty hatches, ladders, etc. . . other things I can't think of now.

Village Life

AI Improvements

Ancient Warfare 2

AnimationAPI

AppleCore

Aquatic Abyss

AquaTweaks

AquaTweaks Compat

Better Foliage

Better Sleeping

Better Storage

BetterFps

BiblioCraft

BiblioCraft: BiblioWoods ExtraBiomesXL Edition

BiblioCraft: BiblioWoods Natura Edition

Boilerplate

BurnCreeperBurn

Carpenter's Blocks

Chisel

chunkgen

CodeChickenCore