I look at mods by what they provide me. I have several criteria that I look at when deciding on mods. Having said that, here's some of the mods I never want to make a pack without:
1) Tinker's Construct. Honestly, this should be the new vanilla tool mechanic. Iguana Tweaks had its uses as well, I liked the idea of replacing tool parts so you don't have a dozen outdated and mostly worthless tools hanging around.
2) AE2. This falls under the heading of 'inventory management'. It's not as LOLPWN as it used to be, but it is still enormously useful. Just cheat in the presses.
3) Mekanism. This is my to-go for tech mod. The energy rebalance is still a WIP so it's not too useful now, but you can still use it quite efficiently. And hey, up to a 5x ore processing, with a delightfully involved and complex process.
4) Thermal Foundation. Note I said foundation not expansion. This, and CoFH Core and Libs, is a great way for a mod pack dev to custom oregen. It's my personal opinion that if you think machines are not expensive enough, the solution is to tweak oregen to be a bit more rare rather than use minetweaker to make the recipes more expensive. Mind you, thermal expansion is still a great mod, but it isn't at the top of the list any more.
5) Blood Magic and Botania. These are my to-go mods for magic. Even without certain... synergies... these are great magic mods because they don't automate very easily. It really gives you a feel of incompatibility between tech and magic that I quite approve of.
6) Pam's Harvestcraft. While I wasn't originally a big fan, with the new worldgen using gardens rather than umpteen bazillion seeds out of grass, I've found that I have become quite enamoured of this mod. It gives you something to eat other than baked potatoes, and re-introduces food as a complex task. Goes well with a nice Chianti and Hunger Overhaul.