What small handful of mods would you really like to play with? That may point you in one direction or another. Currently, I'm working with a pack that includes Buildcraft (minus the Quarry), Reika's mods (sans Chromaticraft), Applied Energistics, Progressive Automation, Railcraft, Steve's Carts and Tinker's Construct for Tech. None of the other common tech mods are present just to force me to do new things. So NO Ender IO, NO RFTools, NO Thermal Expansion, NO MineFactory Reloaded, NO Mekanism, NO Draconic Evolution, etc.
I've opted for Mystcraft instead of RFTools this time, as I have enough tech going on right now. I'll just deal with that mystcraft profiling peeve of mine and try not to fiddle with the modlist too much.
I'm using Dimensional Anchors instead of Chicken Chunks because I *think* the Dimensional Anchors behave a little better.
I do have Thaumcraft (and various addons) installed because I think it's more generally robust than the others and I really don't want a magical kitchen-sink.
I was tempted to add Archimede's Ships after seeing
@The Mobius Archives playing with them, but decided against it. It still seems like an easy shortcut to flight mode and I'm avoiding that.
I have Special Mobs, Special AI and Zombie Awareness included to make my life interesting. Random Things is also present because the Blood Moons make things interesting without going Epic Siege Mod all over the place.
I've used TooMuchLoot to remove a lot of the Mod items from showing up as chest loot, and I'm using VoxelMap in hopes that it won't be as resource intensive as some of the other minimap mods. If I have to, I'll ditch the minimap entirely.
Twilight Forest is in, Forestry/Binnie/Gendustry/Natura/BoP is not. I don't feel the need to have bees in every single pack when I rarely, if ever, use them in any meaningful way. Usually by the time I've advanced enough to make the bees interesting, I've already got the infrastructure in place to make them irrelevant.
I considered ATG, ExtraBiomes, RWG, Highlands. Ultimately just decided to go with default worldgen. I also did a LOT of testing with CustomOreGeneration, but couldn't get it to do exactly what I wanted with respect to areas significantly higher than sea level, so I took it back out.
I've dropped my "almost always added" mods from the pack: Roguelike Dungeons, Recurrent Complex, Better Dungeons and Ruins.
Usually I find myself taking advantage of knowing exactly how these structures are formed and how to best loot the dickens out of them with minimal risk to myself. Lots of reward, minimal risk. Fun for the short term, leads to boredom very quickly.
Chisel (the one with tterrag1098, not Chisel 2) and Carpenter's Blocks are present, Ztones was pulled because of the Totem Tool. Yeah, I could probably have just MineTweaked or TooMuchLooted it out, but ... nah.
For farming, I have Agricraft, Pams Harvestcraft & Hunger Overhaul going on. With a side order of Mr. Crayfish for some neat IKEA-like stuff.
Project Red, Expanded Redstone, Steve's Factory Manager and Red Logic are in because, well, I'm a nerd.
Custom NPCs are in because they just work the way I want them to. No fuss, no muss.
There's a bit more, but they're mostly for convenience or optimization (a la FastCraft, for example) - stuff that isn't strictly necessary for everyone.
I don't think any FTB pack really covers what I listed, and it would require more tweaking than anything to get one to a state I liked, so making a pack for myself was my best option. TomeWyrm seems to go a LOT farther afield when it comes to mods, so his pack probably looks pretty drastically different than mine.
I also tend to work on my own mods and insert them into my pack with varying levels of success. Mostly good, sometimes world-destroying.
So, what mods tickle YOUR fancy?