I would either have to choose NEI, because I never remember the recipes (and the cheat mode is useful in a pinch... oops, did I say that?) or Rei's minimap. All the machinery is cool and all, but I tend to build a quarry only and find everything else way over my head as far as engineering is concerned. I keep seeing people saying they can't play without RedPower, which I don't quite get. As far as I can tell you have to have an engineering degree to make the things RP is capable of (circuits? really? as in computer circuits?). I've been playing Magic World, and Thaumcraft confuses me because it all seems to random as far as tackling the research. MystCraft is an interesting diversion, but I tend to try a couple of worlds, then give it up. No I'd hae to either choose NEI or Rei's, but of the two I can't really decide... Hmmmmm... ok. NEI.
Honestly, I'm going to have to go with factorization after playing with it. It reacts with enough vanilla blocks, adds interesting things to do with items and ore, barrels, and even wierd crazy armor. (Piston shoulders ftw!). The more I'm using it, the more I'm enjoying it.