HTC 8xt was first. It has 8.1 and not upgradeable to 10.
Next came Samsung Ativ S Neo. It also has 8.1. Some have tried the 10 insider preview with mixed results. Last windows phone from Sprint was the Lumia 635, comes with 8.1. And Sprint Spark Fri and LTE.
I have both the 8xt and Lumia 635, each phone has it good and bad.
I have been running the 10 preview on my 635 since the beginning. Microsoft no longer support the 635 with 512 ram on 10. Although I do have the latest 10586.338 version. It is slow though as the 512 ram hold it back. The 635 runs very smooth on 8.1 though.
I use the 635 on my account but still use the 8xt for everything except phone and texting.
None of the three are high end phones but work well if expectations aren't high.