is the NT510 the most popular one out of the Foxwell ones?

Yep.

why is this?

I don't know anything about the scan device you are asking about, but if your interest is to read OBD II Codes, then look into an ELM-327 Module on eBay or Amazon that works with your phone/tablet, then get the OBD Fusion or TorquePro app from the appropriate app store. You will be into this project about $30 and have an app that will give you the ability to build logs of sensor inputs and other things. You can pull codes, reset the computer, and build log files that can be exported into Excel so you can make graphs.

According to foxwell its like getting their gt80 which is $1500 for $149 if its still on sale. The nt510 has the same software as the gt80. So for a particular vehicle its the best one they have.

According to amazon its the best seller of the upper foxwell models.

I don't know what other criteria to check against when I answered "yep" to it being the most popular.

is the NT301 any good? it's half the price of the NT510.

That's a generic scanner so a $14 to $24 generic would be the same as that basically. It has no bmw specific abilities.