This is a really tough question, as so many of the mods we use go so well together (also, once one starts adding mods it gets easy to add more and more and more!). I think Forestry. I'm in the "I love bees" camp, so I think that's what I'd choose. Only thing would be I wouldn't have any pipes, because of no Buildcraft... hmm. Maybe Redpower 2 (when it's out). It's one of the mods I used most extensively when I played that other modpack and the 1.2.5 FTB (not the challenge map obviously). Aside from the amazing technological items (enabling tree farms, various other farms etc), microblocks make building more interesting, marble and basalt are blocks I like to build with, so yep, that's the one I'd go for.
TLDR version: Redpower 2 when it's out.
TLDR version: Redpower 2 when it's out.