Always Nice to Have
Chisel - I love variety. This adds many varieties of common and special materials, just for aesthetics.
Bibliocraft - Furniture. Really brings a room together. Includes shelves for out in the open storage.
Ender Storage - A chest and bag thats let you access the contents from anywhere.
Iron Chest - Upgradable, progressively bigger storage chests.
Twilight Forest - The new progression system is crap (don't tell me where I can't go!) but the interest this adds in great. Ore hills are wonderful resources balanced with danger, and the bosses are always fun to battle.
Roguelike Dungeons/Ruins - I do like exploring. These provide the opportunity.
Hardcore Ender Expansion - Makes the End interesting, adding incentive to explore.
Mariculture - Makes underwater interesting.
Natura - Makes the nether interesting!
Biomes O Plenty - Makes the overworld interesting!
Project Red - Adds the classic gemstones and some nice world generation.
Railcraft - Good way to get steel. There are trains apparently. Requires Buildcraft... so quarries by extension... although the Ender Quarry is better.
Minefactory Reloaded - Despite the foul taste in my mouth from the Tree of Darkness... has some really useful machines for farming.
Apple, Milk, Tea - Adds more food and some appliances.
Useful Food - Allows you to make simple sandwiches from vanilla bread and meats, so in that regard, better than Pam's.
Dense Ores - Pockets of more ore drops... so saves a little time.
Big Reactors - Ultimate power! Generates and awesome amount of it.... and they don't explode.
Draconic Evolution - Endgame power generation and more dragon fights.
ICBM - There are just times you wanna nuke something, y'know?
Applied Energistics - Surprisingly, I CAN do without this one if there are other storage options. I don't particularly care for the slow start with the seeds growing in water.
AmbivalanceFossils and Archaeology - Seems like one that requires alot of work and space with little unique payoff.
Artifacts - Gotta say, pretty unbalanced. Also... seems to hate world generation, resulting in corruption.
Chemcraft - I have not found a compelling reason to peruse this mod. It seems like it might add too much, and I don't like unnecessary bloat.
Terrafirmacraft - My experience with this one has been largely frustrating. I can appreciate it at times, but it just seems to make tasks tedious.
Chococraft - Liked the old FF games, but this doesn't grab me. I'd rather have magitek armor or the FF summons, frankly.
Computer Craft - I keep my coding and adventuring separate. May be great, but I don't feel like learning a new language for the sake of a mod.
Advanced Genetics - Some experience with this, mostly due to becoming blood brothers with that guy that once... but it's alot of work. I can't say it's not worth it, but I might not be incentivized to try if it takes too long.
Morph - It won't deter me from a pack, but I will not use it. It doesn't fit... that's all. Why can the player do this? It doesn't use any resources, so it seems a bit overpowered.
The Dregs of Mod-manateeBelow are some that will cause me to avoid a modpack if they are included.
Galacticraft - It's about the journey... not the destination. It's a great deal of work and very little payoff. Each moon/planet is practically a palette swapped version of the one before. Lots of work to get there, and the only reason to come is to leave.
Infernal Mobs/Grimoire of Gaia/Lycanites Mobs - Each adds many monsters (mostly hostile) with hit points out of proportion to the scale of the game that drop alot of clutter items. They attack unprovoked during the day and night and usually in groups.
Orespawn - As above... only massively, ridiculously more out proportion. I thought the epic rules for D&D were bad, but this is orders of magnitude greater than I ever thought possible.
Metallurgy - Adds too many ores, muddying the waters and making common ores harder to find. Also, alloy too easily in the TC smeltery.
Chocolate Quest - This one is a mess. I like dungeons, but these are ridiculous. More hordes of monsters, same combat as ever. Might as well just reskin a zombie.
Gregtech - In the name of needlessly complicating things, I am Captain Gregtech!