I liked the original Most Wanted, there was an actual storyline to it instead of just being thrown into an open world and taking out racers. Granted, the storyline may have been a bit cheesy at parts, but it was enjoyable.
Prostreet was different, but enjoyable..I really liked the drag racing. Shift was pretty much a rip off of Grid or Ridge Racer if you think about it. Again, enjoyable but I preferred Forza when NFS went towards that closed track type of racing.
Forza Horizon combined the best of both worlds for me, open world racing with the graphics and style that Forza is known for.