Huntsville to Charleston, SC

I would take 431 down to I20 and then east to Columbia SC and then down I 26 to Charleston.

I guess the operative word is 'best'. Lot less curves and hills etc if you head down to Birmingham and then east to Atlanta and on to Columbia and then 26 to Charleston. That said, I find I-20 one of the most boring roads in the SE SA - almost as bad as I-10 in the Florida panhandle. I suggest you get west of Atlanta before overnighting. I-20 through Atlanta is best taken straight through. You can see the state dome covered in Georgia gold as you pass downtown. Try to go through after rush hour (maybe after 10 AM) and before 3 PM) The traffic gets easier to deal with after you get east of Conyers. Overnighting at Madison Ga (US 441/129) there is a couple of truck stops if you want to dry camp or if you go south a couple of miles from the interstate there is a RV park on the left. Not a resort, but adequate.
From there to Augusta it is just a drive. Not a problem even around rush hour. Cross the river and it is about an hour to Columbia and I26. Cannot recall which exit will cut across to I-26 from 20 but the interstate intersection is only about 4 miles from the cut through we always use.
Hope you have a good trip. If you can find Shem Creek in Mt Pleasant across the river from Charleston, then look up Reds Ice House and have seafood meal with their home made fried potatos and a cool beverage on the deck overlooking the creek. Have one for us, we love the place!