AdditionalPipes, quite handy for the times I have to use crappy buildcraft pipes.
advanced repulsion systems, haven't tried yet.
Advanced machines, these are slower after 10 or so overclocker upgrades, so only for mid game, still, quite handy then.
Advanced Power Management is useless to me, though the adjustable emitter might be nice in Creative mode.
buildcraft, I hate BC pipes, and wish I could dump it, but there's too much extra useful stuff in the mod.
ComputerCraft is amazing, but I could live without it thanks to RP2.
ee3 isn't complete yet, so can't really judge.
extrabees, seem nice, but my bees are morons. Yes, Ms. Meadows queen, you're in a plains biome, in an apiary surrounded by various flowers, it's sunny out, and you can't find any flowers. :/
ExtrabiomesXL, pretty. Could live without, but wouldn't want to.
Factorization barrels seem cool, but I've not used them much. TubeStuff Black Hole chests + RP2 sorting works more for me, though more expensive materials.
forestry is a cool mod, and I use bits of it, but sadly uses Buildcraft. Happily it allows electrical engines, which remove much of the pain.
GregTech. I currently have this disabled, if I can figure out how to stop it from changing any other mod's recipes I'll enable it. It adds a cool endgame, but the massive grind added in no way balances anything. It just means I spend longer setting up and less time building, and where's the fun in that?
IC2NuclearControl, I don't like nuke plants.
iron chests are great, but TubeStuff's Black Hole chest is better if enabled.
LogisticsPipes make Buildcraft pipes useable.
MineFactoryReloaded Safari nets alone make starting animal farms much, much better. Not necessary, but very nice to have.
miscperipherals Bunches of useful stuff I've not used.
mystcraft is pretty, and lets me add things to the worldgen without regenerating the entire world, just make a new age!
Nether Ores isn't very useful. Still, it gives something to do in the Nether.
OmniTools are handy to save space, but not really necessary.
Portal Gun is great fun, and good for long distance travel, but I can live without.
SoulShards is handy, and makes creation of xp/item farms easy, but not as necessary as RP2.
StevesCarts. I've not really touched this much, yet.
Thaumcraft3. I've not really touched this much, yet.
Traincraft, I'll probably never use this. If I want to play Open TTD, I'll play Open TTD.
Transformers. Haven't touched it.
twilightforest is pretty.
xycraft has pretty blocks for building.