Not what you wuld think of as an expert, but wool is good. So is down. I pack both.
I never bring synthetics, except for a light fleece pullover.
A good wool sweater for packing is probably not made. Merino wool is not very good. When knitted, it produces a fine woven, thin layer. Great for long johns, not what you want for a warmth layer. Nor, do you want a 3/8" thick layer of sweater made of all guard hairs from a wool batch. Pretty heavy, and not too compressable. Something inbetween is good. I have two that I use. One is from Lands End, the other is from some no-name. Look for a fine cloth with some fuzz. They weigh 8-1/2oz and 10-3/4oz.
OK, you need a base layer. Merino wool is good for that and highly recommended. Long sleeves keeps bugs off. Over that you can add your sweater. Worn alone it works to keep you warm and ventilates well while hiking. And, it sort of does double duty. Stop hiking, and the fuzz will hold your heat in, making you warmer. Hike at 2mph and it ventilates well, not allowing you to overheat (this depends on other conditions..., of course.) It packs up to about the size of small cantelope without compression. If it starts getting cold, slip something tightly woven over it. A light fleece works. Now it will hold heat in all the air pockets pretty well...a good insulation layer.
Overall, a wool sweater is good in every category: warmth, cooling/ventilation, insulation capacity, warmth while wet, compressibility, etc. It is NOT the best in any one, though. You can easily find other stuff that will do better in one category at the expense of another. Overall, it is excellent, because it does everything pretty good. BTW, a wool sweater makes a fair pillow, too.
Down is better at insulating. But it requires a fine woven cloth (down proof) holder. It is much warmer...too warm for hiking, generally. And, if you let it get wet, it is not good at insulating. Excelent for compressibility. I carry a light down sweater for around camp and at night. About 11.5oz. Best insulation per pound carried.
Synthetics should never be compressed. It will put permanent compression into the fibers. But these have excelent wet warmth. Again, it will depend on conditions and how you use it. I never carry a synthetic jacket, relying on my light fleece, instead. It does not compress as well, but blocks wind a bit better. Good weight, but large volume.