If you what the lightest, most compressable, warmest fleece then there is only 1 answer.
Regulator fleece. It comes in R0.5,R1, R2, R3, R4
It has the best warm to weight ratio.
Initially invented bye Patagonia but now in use by Haglofs too
The Jacket Harry bought in Anchorage is lowe alpine R2. Very good price but not as nice and high tech as patagonia or haglofs.
For examples look at Patagonia Alpine fur, Haglofs Siberia jct