In-game I'd have to go with a Minolta Toyota. They're one of the only things I have to worry about beating me or B-spec Bob. And the blacked-out headlights are pretty scary.
In real life... 2011 Dodge Charger State Police car
The only time I have felt this threat (since I very rarely play online) was in the Historic cup. I was using the Camaro RM. The AI don't overtake very well, so I used this to my advantage to jump to the front and build a small lead on lap 1. Then I'd see it overtake second and launch itself towards me once again with its white livery. The Toyota 7 race car.
I haven't raced online at all - in fact since I booted up this game mid-February I have played the original disc strictly offline. The less-than-intelligent AI is my only competition so far, (though apparently better-mannered - they never drive backwards.) They can be irritatingly persistent though, and when I'm fighting my way to the end in the Dream Car Championships, one car I hate to see in my rear view mirror is that smokey purple .......grrrrrrrrrrr ........you know the car I mean don't you?
For me, it's gonna be the Pajero Evolution '03 that has great road presence, especially at night. I'm not threatened by it, as I drive one in GT5 but most of the time, I feel like the AIs are threatened by it. The car rides high and is bulky, too. It feels like it wants to hunt and beat people by running fast, but sometimes, it is slow.