NEI, without it I would be completely lost.
Railcraft for content, it's just so much fun building complex transport systems, I still don't understand why you would miss that by teleporting stuff.
Same. Not even close. I like the look, feel and sound of carts, all of the multiblock structures, the look of the various decorative blocks, the sounds of the rock crusher (for example), I really just like it all.
And I really like the fact that you have to think about things ahead of time, much more so than on some of the other mods I've tried which, for me, border on ridiculous with some of their machines.
I just wish it had a machine like Forestry's Carpenter as it's one of the only things I really use in Forestry even though it's a nice, well thought-out mod.
Buildcraft, too, of course.
IndustrialCraft sounds a lot more interesting now than I found it to be in 1.2.5 but, aesthetically, I don't like it at all and don't think the machines and such fit my view of MineCraft.