Realistically?

'27 T roadster, Hudson 308ci Twin-H I6 build to NASCAR 7X spec or a cheapie junkyard smallblock Chevy
'53 Hudson Hornet, Twin-H 308ci I6
'56 Olds 88, preferably a 2d HT
'57 DeSoto, any model, preferably 2d HT or a wagon
'58-'61 Chevrolet, any model, preferable 2d HT or convertible
'59 Buick, any model, preferably 2d HT, convertible, or a wagon
'63.5 Ford Galaxie 2d fastback HT

Once my Focus is paid off, no more late-model cars for me.

98-2004 BMW M5

Realistically 98-2004 BMW 540i 6 speed manual.

Excellent Question
 

I'm asked this question all the time, and it's hard to answer.

Ideally, I'd like an aerodynamic hatch or wagon with low displacement and a manual gearbox suitable for efficient highway use. It has be mass produced, easily serviced, and the method of propulsion readily available.

Goal: low-emissions, everyday driveability, 4-passenger capacity...

I've often imagined of the Civic GX with a home Natural Gas filler unit and plug-in hybrid IMA drivetrain with 5-speed manual (if it existed). Out of the box, the Fit Manual looks like a good replacement for the 'Teg.

Reliability notwithstanding, if I could try any car in the world, the UK offers a few Diesel options...

-UKDM: Citroen C4 HDI (92 bhp) 5-door. Diesel, low-emissions, good aero. Smaller Citroen hatches aren't suited for highway use.

https://pic16.picturetrail.com/VOL698.../267131332.jpg
Rumors of the diesel-electric hybrid sound good...

-UKDM: Honda Civic Hatchback 2.2 i-CTDi Sport 5-door. A Diesel Civic with a Fit-like engine, interior, and emissions profile...

https://pic16.picturetrail.com/VOL698.../267133818.jpg

-UKDM: Hyundai i30 Hatchback 1.6 CRDi 5-door. Looks like a Diesel Elantra 5-door with extras and good FE/emissions.

https://pic16.picturetrail.com/VOL698.../267135302.jpg

-UKDM: Toyota Auris 5-Door Hatchback D-4D. Why don't we have this here?

https://pic16.picturetrail.com/VOL698.../267137096.jpg

All of these have an optional Diesel engine that have lower CO2 emissions and higher FE. Assuming these could be modified for our stricter Diesel emissions standards would be great. Otherwise, decent gasser engines can be optioned.

There's still a horsepower streak that needs to let loose...

https://pic16.picturetrail.com/VOL698.../233867418.jpg
This looks like fun for the weekends :D