My favorite concept has to be Nissan's 1985 CUE-X.
I love how flat it looks.
Mechanics are as Nissan as they get;
300hp VG30DETT with four wheel drive and four wheel steering.
My second favorite is the 1999 Nissan 240Z Concept; even though I find it quite unattractive;
The reason I like it even though I think she's fairly ugly is because of the car it would have been. This was a fully functional vehicle based upon Nissan's S-Chassis, powered by my beloved KA24DE tuned to put out 200hp. That in my eyes would have been the perfect inexpensive sports car, and the car I would have bought if she had been put into production.
My third favorite is probably Nissan's Trailer Runner concept from 1997.
That ground clearance.
This thing might have been a Dakar legend.
Okay... so I should probably post something that isn't a Nissan.
Uh... how about the Zis-112?
Certainly unique. It was a Russian race car, and it took 3rd place in the 1956 USSR Championship.
Then there's the I.A.D. Alien
Kind of cliche' at this point... but I still like it.