My personal mod list:
Somnia - makes beds work much better, actually simulating the world while you sleep.
Smart Moving - lets you crouch, sprint, swim like a pro, and hoist yourself up onto 2 and 3-block high ledges.
Peaceful Pack - adds fossils for bones, flax for string, sulphur and niter for gunpowder, and slime pools that all work on peaceful. (I just use it for more plant or cave variety, I play on Hard.)
Mystic Ores - Adds about 10 new ores, rare and common, each with special properties.
Mystic Ruins - Adds randomly generated ruins and sometimes custom dungeons, as well as vines that generate under leaves and underground.
Pathing Activated - Gives all monsters a sense of smell and hearing, as well as improving their pathfinding. You can tweak everything to your liking.
Crystal Wing - Lets you craft a special item that can teleport you to your spawn point a few times before breaking.
Web Generation - Generates webs in caves that can be broken with a sword, like Terraria.
Survivalism - Gives you energy, food, and water meters. Also limits you to 3 lives before you're locked out of your save, making every step count.
Matmos - Adds ambient sounds to the world, making the atmosphere interesting for once.
Armor Stand & Shelf - Lets you display your items and armor.
Balkon's Weapon Mod - Gives you a variety of weapons to craft, each with different properties, including crossbows, daggers, spears, battleaxes, and flails.
Thinking of installing:
Sound Dampening Mod - reduces sound travel through solid blocks and adds fancy effects like echoing, reducing annoyance and increasing immersion. (not sure if compatible with Matmos)
Mo'Creatures - Adds lots of new creatures to the game. (I've tried this before and love most of the animals, but I'm not sure if it's compatible with Pathing Activated)
Baby Animals - Adds baby versions of all the animals that can be tamed to raise a farm. (Also not sure if compatible with Pathing Activated.)
Let me know what you guys think.