Pulsar GTi-R is one of my favourites but unfortunately that's stuck in GT1.
In game I like the Silvia 240RS though. And obviously all of the pre 80s Nissan's are very good. Not very fond of the 80s and over Skyline series but that's not to say I dislike them, my personal favourite is the R31.
The GT class rules were changed to prevent manufactures from using the many loop holes. Thus the R390 was killed, only 2 were built for road use per GT class rules anyways. Like others they moved over to the prototype classes.