I have the atom lt by Arc'teryx,it works really well and is a nice piece of gear.
I also have used my friends nanopuff patagonia jacket. It is made with 60g primaloft one insulator. It worked pretty nicely.
But my favorite midlayer, and the only one I ever use anymore is the super down jacket / pant by KUIU. This is a synthetic down that only weighs 10 oz and is warmer than both mentioned above. It is made with the finest polish down feathers that have been treated to be water proof. The fabric comes from Torrey, who also make jackets for a company called KJUS that sell for around $1,000 each (the kuiu super down is $249.99). The jacket is made with new fabric that stretches and allows for a full range of movement which the other 2 jackets mentioned above did not. And if you are looking to spend less money but still get a top of the line product, look at the KUIU spindrift jacket. Made with 80 gram primaloft one (20 grams more primaloft one than the patagonia nano puff). It is also only $139.99.
Check out their website. kuiu.com
They are a relatively new company, if anyone else on this forum has every tried Kuiu, they can vouch for how great this gear is.