InfiTech2, because GregTech is the greatest tech mod ever. Jason McRay has done a wonderful job with eliminating as many recipe "exploits" as possible, meaning it is truly a hard-mode type of pack, but it is not at all unfair or cruel. There's no infinite water, for instance, however you can use the Railcraft Water Tank or once you achieve machines then you can make the Reservoir for all your water needs. Hard, but fair, a perfect example. BloodAsp also gets a huge credit for taking the unfinished "GregTech v5" mod and with the permission of the mod author, adding to it and making it as complete as possible, calling it "GregTech 5 Unofficial". It is simply wonderful, he even added fusion reactors as the end-game goal (which every pack should have, an end-game goal, but many do not).
Finally, in case you don't know, GregTech uses an entirely unique and (in my opinion) very realistic ore generation. There's not just the 6 basic vanilla ores found in every modpack, oh no... GregTech has probably 100 ores to find (no easy task!!), to mine, and to process in various ways and combinations.
That's my opinion, since you asked.