Once you get more than 1 or 2 small content mods, I would call NEI (or similar) essential. Beyond that it is all a mater of personal preference, & opinions vary wildly. A large, general pack is great for figuring out which mods you like & which you don't, but once you know, I think a smaller, tailored pack is better for long term gameplay (or relatively short play with something like AgSkies or ME^3).
Unless you are trying to set up a public server, don't worry about making a pack that will please everyone, because you can't. Think about the specific things you want to be able to do & how easy or hard you want it to be to get to that point, then compare the mods that add those features.