The one that you can comfortably shoot accurately and develop the confidence to seal the deal in the moment of truth.
For me it's a .270 Win, but a .243, 25-06, 30-06, etc, etc, etc, ad nauseum will get the job done. Don't buy into the magnum hype. Nobody needs a 300 Win Mag to blast diminutive wt's over a feeder at 100 yards or less. A 30-30 will get that job done just fine. Now, if it's your choice to blow deer to kingdom come with a magnum at 100 yards, then by all means. But it's not necessary.
Hunt near Freer