Started listening to them around the "being there" / "summerteeth" era. I think those are their greatest works.
However some of the best stuff is the Uncle Tupelo stuff, especially "Anodyne". The fact that it's from a genre I usually don't like (ie country music) makes it even more amazing. Of course you must mix the country music with punk to make it interesting. Tweedy and Farrar were so good together, but it's a shame they couldn't work together.