Hard to recommend a bag without specifics. What is your idea of ultralight? How far below freezing do you want to go? Are you a warm or cold sleeper? Will you be in a bivy or a tent, or out in the open?

I can take my 16 oz. WM Highlite down to freezing IF I'm wearing warm clothes and a hat in a tent on a good pad. And barely. But I'm a cold sleeper and YMMV.
If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.