Ah, nice to see attention to detail; there must be some things inimical to both these threads (those in the GTPlanet GT6 Tuning Forum) AND the characteristics of those who see tuning in GT6 as a legitimate realm for their quasi-scientific curiosities. These are, after all, what lead me here.
I suggest the Lancia STRATOS and the classic Yellow Bird for these reasons:
1) unique handling out of the box (is. Completely stock)
2) may benefit from deeper tuning explorations than those that seek to (merely) get it to go as fast as possible on x tires at y PP on z track, etc. (PP rounded to the nearest 50 and so on)
3) Schadenfreude, to be perfectly honest