Okay guys, let's have it. I'm especially interested in what Jonathan, Glasswolf, and MazdaMan will have to say.
You have no budget. If you want to spend $50,000, go for it. It would have a perfect mix of SQ and SPL, but weighed a little bit toward SQ. If possible, put the approximate price of your components, too.
Any type of car you want, too. This should be interesting!
You also have to consider that the "perfect"(non-existant, by the way) system also depends on your vehicle. Different vehicles offer different acoustic properties. While you should choose components based on personal preference and build/sound quality (or SPL if you're an SPL guy) you have to consider how far you're going to go. Electronics don't rely on the vehicle as much, choose head units on sound quality and ergonomics, choose amplifiers on reliability, SQ, and the design. A well built amp will hold up better in the car environment due to a better power supply and heat management. Some speakers work very well w/ factory locations, some plain out suck. Just like the 2 way vs. 3 way components issue. If you can't ->properly<- accomodate a 3 way system, it'll give worse results than a 2 way will. It's all in the application. Some vehicles work well with a sealed box, some have very little gain and a ported would work better. My ideal vehicle wouldn't even be SPL inclined, yes it would get loud due to sheer power (for headroom) but SPL wouldn't be what I chose components upon. The good thing is that most top end SQ subs also hit hard, because the subs must keep linear for a large range of excursion to maintain low distortion(done with strong motors and a well though out motor design, such as Adire's XBL^2 and other underhung designs, etc.), and this usually ends with a long throw (equaling SPL) and a well controlled, fast transient response, also helping SPL. But we did post this not too long ago, my choice of components would be along the lines of the following: Alpine F#1 Status, Nakamichi CD-700 head unit
Dynaudio or Rainbow components, kick panel location. Possibly have main tweeter up top, depends on the car though if it'll work well as far as phase and acoustic properties (if it'll sound layered or not)
Zapco amps and signal processing, EQs, and active crossover for components
Probably ID Max 10" or 12" subs in aperiodic membranes in the package tray of whatever vehicle I choose.