Perhaps not the best possible car as it is not entirely practical but it would be great!
Take the Z4
Mechanical:
Give it the 268hp direct injected N52 in the euro-spec 330i (not a huge change, still an N52B30)
Give it non RFT tires which will hopefully fix the tramlining and general lack of on-center feel.
Fix the transmission programming in D mode.
Although a DSG would be great.
Cosmetic/Interior/Niceties:
Give it BiXenon HIDs with auto swivel
New generation navigation and iDrive bits and such. HK audio and sat radio, etc etc.
Sport seats with thigh support.
Better cupholders.Done! That's probably idea for me as far as hooliganing around.
As for the best practical / DD / roadtripping car, I am really liking the EX35 ever since I test drove it. But a few changes:
New 3.7L VQ engine. Or might as well go whole hog and gimme the 5.6L V8
New 7 speed transmission (they already have an EX37 in Europe anyways....)
Stretch it about 2 inches for the rear seat to not suck as much. 2 inches hopefully won't ruin the proportions.
Slightly less steering boost.
Move the rear turn signals up from the bumper into the brake light cluster. As it is they are positioned SUPER low and I doubt people in taller SUVs would see them.