In my opinion, Mekanism for the mech, smartmoving because minecraft is not the same after you used it, enviromine for the physics and air mechanics which are seriously awesome and primitive mobs to make the thing more fun.. After that, everything depends on what you want to focus the pack, but I will never play a pack without those 4 mods again, as simple as that, they give the game just too much for me to accept less.