Everyone says RP2.
I love it, it’s really cool, but I will never sway from my absolute final decision of Industrialcraft, and I'll tell you why.
When I first began playing Minecraft with mods, it was beta 1.8. I had 4 mods, Industrialcraft, buildcraft, Millenaire, and treecapitator.
Later, I added redpower... a good bit later.
But regardless. I had never really played with IC2 anymore, or any mods. Buildcraft was fairly content-less. I'm not sure that it had conductive pipes, I didn't even know about them if they were there. Really, buildcraft was quarries and pipes, and the occasional mining well. Millenaire, I never really did anything with. There was a village near my house, (And a vanilla village, a small one) But I never really did anything with them, they were boring. Treecapitator, I've really never played with since.
But IC2, it was great. It was a whole new adventure, where I was rushing back to the wiki every few minutes just to see what new (To me) thing I could craft. When I began, I knew the macerator, the electric furnace, and solar panels. I was determined to do it right, and kept building. I started by making a little hole in the ground. I made some solar panels and some machines. The details are fuzzy, and I won't tell the whole story, but I spent a lot of time seeing what I could make. I ended up with a half built construction foam castle, a huge rail line, a million underground farms, and a big above ground one, (That floated), with a bunch of outposts, and one huge quarry that had a cool Jeffery-tube like hut for the engines and stuff. All my time was spent exploring and making things, from scanners to jet packs, and just being fascinated with all the amazing stuff. No matter what, I could never let go of the mod that gave me some amazing memories. I've since lost all backups of that world, to accidental deletion, or corruption, or some other things. But IC2 just holds a special place in my heart. RP2 I awesome, but I can live without it. Rather easily actually, even though I miss my infinite cobble supply.
Anyway, everyone has their favorite, but I love IC2. I won’t rag on anyone else’s choices, but there’s my choice. Thanks for reading!
TL;DR IC2 Freaking rules, I will accept no other arguments.