I'll disagree with everyone here (that's my job, haha).
The best t-shirt for printing on the Neoflex is...
the one that you have practiced on the best with, and have wash tested the best.
Printing on shirts is not a matter of buying the best ink, the best pretreat, and the best shirt brand. It's about you. I bet we can take the "best" Neoflex owner here, and they can print better on Gildan 5000s than the "worst' Neoflex owner here could do with a Keya.
Learn to pretreat -- by hand, by machine, both. Make sure your heat press for curing pretreat is calibrated and consistent. Get a good gram scale so you can verify how much pretreat works best on what brand and color garment. You might want to even try two different brands of pretreat and do tests on both.
Learn to work with NeoRip. The TIGERS have done an excellent job setting up NeoRip to work with cotton shirts. It is mostly plug-and-play, but there are optional adjustments in the software for a reason. The more you learn what settings work with your entire process, the better the quality will be. You may even save money in the long run if you can tweak settings to use less ink to get a similar output.
Perform wash tests. Don't just do them as your recommendations will be to customers. Do them as customers actually will wash them. Wash in hot, dry in hot, wash in hot and leave in the washer for a day before drying. Try it with abrasive detergents. Spill some spaghetti sauce on your garments and use a stain remover and see what happens.
I'll agree with the brands listed -- ringspun, quality cotton, without anti-stain features -- will print the best. If you pretreat properly. If you use the proper settings. If your heat press is accurate, and you are consistent with temperature and pressure and humidity and all the other magic involved.
It's not as scary as I make it sound, but expect to burn some shirts. Maybe many shirts. You want a product that brings customers back again and again. That product won't be had by just clicking print and closing your press. Remember: what looks great after curing may not look great after washing a few times. But you can get there. You WILL get there.