So, having quite a bit of experience under my belt with mods, modpacks, and creating my own packs... I'll record some of my thoughts on how various mods measure up.
The EssentialsDamage Indicators - Perceivable consequence! I like to know how effective my hits are besides the vague feeling of "it takes several hits to kill" or "that went down quickly". Seeing the exact numbers gives players a better sense that they are doing well.
Voxel Map - My personal choice for mini-map/big map, and location marking. It allows easy teleportation to markers and is relatively unobtrusive. The death markers do tend to accumulate though.
NEI - Allows you to look up recipes for items. Too useful to ignore.
WAILA - What am I looking at? This tells you. Some additional plugins give other useful information.
Treecapitator - Honestly, this saves so much time. The bigger the tree, the longer it takes, but when you consider the time saved on huge trees just navigating to the highest limbs, you gotta have it.
Fastcraft - The larger your modpack, the more necessary this becomes. Amazing how in its absence the game just comes apart on you with lag.
The Best of the Best of the Best, Sir! .... With Honors!
Tinkers Construct - The somewhat tedious early game gathering of simple materials to build the multiblock structure will pay off well when you get your hands on a hammer. The tools can be modified with materials to make swords sharper, tools more durable and capable of silk touch. (Requires Mantle)
Extra Utilities - The Chandelier and its big brother, Magnum Torch! These items what prevent monster spawning are enough of a reason by themselves... I never liked the look of covering the ground in torches. Also includes the golden bag of holding for on hand storage and the golden lasso for capturing creatures, and the Ender Quarry, a good alternative to the more cumbersome buildcraft one.
Mekanism; EnderIO; and Thermal Expansion/Foundation/Dynamics - Power struggle! Three way tie. Each provides great suite of power generation, storage, sorting, and machines. (Thermal requires COFH Core). Mekanism has the best jetpack and upgradable machines in my estimation. The EnderIO staff of traveling is pretty nifty.
Open Blocks - Elevators. One block, simply crouch or jump to go between them.
Pam's Harvestcraft + Cooking for Blockheads - Pam's adds a great variety to plants and food, wheras CfB provides manageable interaction with the former.
JABBA; Storage Drawers - It's nice to have a way of sorting that lets you see your resources in the open.
Nice to Have (for aesthetics or adding interest)
The Dregs (some to avoid).