I know that ideally, I should keep my ground coffee in an airtight, opaque container.
Currently, I'm limited to one that is airtight, but glass, and one that is opaque, but not that airtight (it's actually a bag with a zip seal).
While I get around to getting a decent new container, which is the best to keep my ground coffee in?
I value sealed over opaque. Go for the sealed glass. Opaque is not really the goal, but rather in the dark. Keeping the stuff in a dark location, even in a clear container, will be fine.
As far as priorities for storage in general, according to the USA National Coffee Association (and others including Blue Bottle and a lengthy missive from The Atlantic), there are four things that are most important for coffee freshness; that is, minimizing exposure to the following:
... in that order, according to NCAUSA (first link above). That is, keep it sealed (minimizing airflow and humidity, most important), cool (but not cold/refrigerated), and in the dark (somewhat less important). The best place to do this is in an airtight container in a dark, cool pantry. If kept in a dark place, you don't even need to make the container opaque. Lastly, the "light" factor is the lowest in priority, so "mostly dark" is probably good enough.
I tend to re-use coffee containers for this purpose (I have a bunch of illy cans such as these that work well, are a good size, and have a one-way valve on the bottom). Once you buy the coffee, the tins are "free"!
