The mod that got me into modded Minecraft

TerraFirmaCraft is a detailed total conversion survival mod with a lot of custom mechanics, and has some of the best crafting mechanics I've seen to date. The 1.7.10 version in particular is so comprehensive that the mod can pretty much be played on its own. Worldgen, blocks, and items are overhauled, and there is even new music and mobs. Unfortunately, the mid/late game gets repetitive and is mainly just improved methods of metalworking. iirc the mod also had issues with insufficient in-game documentation, which was less of a problem when the 1.7.10 wiki was around, but now that version of the wiki is gone. Still, it's a great mod overall that is worth trying at least once for the experience.