I would have to say Nikki. Seeing as she has a Ford GT, you wont leave her behind and she'll stick with you and be there whenever necessary - an important attribute. She also gives you a 10% discount on car purchases and 50% MORE speedbreaker and NOS. Also, she is damn hot.
The REAL DK has spoken
Same Nikki is the best, but I can also go with Samson in most Sprints, and Sal for most circuts..
Depends on the track. If it's circuit go with Yumi, Samson's the man in Sprint and when you feel the need for speed in speedtrap, it's time to haul out Nikki.
Though my personal fav is Samson, becuase I just love to see him ram my competition off the road!!
He's never as good when you get to Silverton