For what it's worth I have owned and put many miles on: the G Wagon; Panamera 4s and A8 (as well as a 430, 458 and DB 9 for fun rides). Experience:
G Wagon - A unique "truck"; certainly sets one apart; the handling is almost scary on corners; be sure to take a long test drive and drive in sunlight as the flat windshield glass imparts an odd image at times; inhales fuel; is great if you plan on serious offroading - if not you are making a "fashion" statement.
A 8 - excellent electronics and intuitive controls; very performance oriented for a large sedan; and it is a sedan.
Panamera S - Best combination of a practical DD (it does actually seat 4 humans) which is fun to drive and has sufficient performance so you can have some fun. Would be my choice. FYI, drove it all year in Wisconsin weather with all season tires. Excellent performance.