Ignoring for the moment the likelihood that ponies can’t actually drive, this Chinese-vended Lamborghini Gallardo seems perfect for Pinkie, even if it’s way too fast for the bucolic streets of Ponyville:
Or maybe it’s just that Pinkie Pie, in common with most Lamborghinis, pays little attention to the laws of physics.
Beyond this, we could place Applejack in an F-150, Rarity in a Lexus ES 350 (a Camry would so not do), Fluttershy in a mid-1980s Volvo 240 (polite, safe, and unassuming), and, well, we’ve already seen Rainbow Dash’s trick Mustang.
That leaves Twilight Sparkle, and to me the choice seems obvious:
Methodical, comprehensible, and no frills.