Polytec 301 is the obvious winner on that page because it HAS a thermal rating and the number is, "3".
(Higher number is good.)

Would you like to convert BTU inches per hour, per square foot, per Fahrenheit into Watts per meter per Kelvin?

Your right 301 has the better thermal rating than the others.

They don't show a thermal rating for the Polytec 437; but clearly state that 437 is typically used for thermistor potting. Why is that?

 

They don't show a thermal rating for the Polytec 437; but clearly state that 437 is typically used for thermistor potting. Why is that?

Why do manufacturers not provide the most important specification for their product? You will have to ask them.