I have an Goodman HPTD 18-20 could you tell me the best compatible thermostat to replace it with to match wiring and please tell me which other letters corresponds with my Goodman. Y, COM, R, G, W2, E/W3, O, B. Could you tell me which other letters would correspond with E/W3 and COM.

A 2-stage heat pump thermostat would be required, but a 3 stage would be dandy, if you can find an affordable one. COM is the other half of the low voltage power supply (R). May also appear as "C" on some thermostats and/or equipment. E/W3 is a third heating stage, which can be connected to either "E", a late-stage "W-", or jumpered to "W2".