IF, I were to have only one gun, it would probably be the 30-06! I do not have only one gun and I DO NOT, and never will have a 30-06!
That said, a 30-06 IS a great cartridge, and the .308 Winchester is basically its equal.
But......
For deer size game, anything from .243 to 7MM-08 will do just as well, with less recoil.
For larger game, (especially at longer ranges) the magnums will do it better than the '06 or the .308!
To me, the .243 is a perfect duel game cartridge! (Varmints/deer)
The .308 and '06 are perfect duel game cartridges too! (deer/elk)
But, there are better "specialty" cartridges for all needs!