Galacticraft, Tinkers' construct, and Orespawn. (In that order) Links to the mod pages are included.
Galacticraft - In space, nobody can hear you craft... Galacticraft allows you to travel to other planets, build homes on those planets, and even make your own satellites!
Tinkers' Construct - This mod allows you to build a smeltery, get double ores, and make so many more tools!
Orespawn - Orespawn is a combat based mod that adds more (much harder) bosses, dimensions, stronger enemies, swords that do 750 damage, a pet that actually grows, (and eventually becomes the second hardest boss, unless you leave that config option alone) and ores that can be turned into spawn eggs. (Hence the name)