This is a fun question. NordicTrack makes some great machines, my favorite in fact. I guess there are two ways to answer this. First, which NordicTrack treadmill is the best overall, and second, which NordicTrack treadmill is the best for its price.
First off, the X9i incline trainer is definitely the best NordicTrack treadmill out there right now. It can incline up to 40% and decline up to 40% and comes with all kinds of goodies, like iFit and a touchscreen web browser. I have one in my home and it is the treadmill I use.
The best NordicTrack treadmill for the price is probably the T5.7. If you have a little more and can go up to $1,000, I would go with the C900 Pro. But if you are on an extreme budget, this $600 treadmill is a steal. Not the best motor, not the largest walking surface, but a good all around treadmill that works for the average person.
I hope that answers your question and helps you find the best NordicTrack treadmill!