With the cartridges you already own I'd spend some money on soem good optics instead.
If you really MUST purchase something the .308 would make the most sense for practicality, and the 7mm/08 or a .260 or 6.5 X 55 would also fill in the nitches of your battery. But with the 30-30, .243, the .270 and the 30/06 as well as the 7mm Mag, you are pretty well set for whitetail.

Buy a better scope or a good set of binocs instead, unless you MUST have another gun.