"Crack in a Hard Heart" is a good one; definitely overlooked it. Gorgeous AC Farley art.
One series of themed back-ups are the vignettes by Will Tupper, which were scattered about several issues and at a glance you wouldn't recognize them as an actual series.
http://tmntentity.blogspot.com/2013/...ll-tupper.html
They're a bit uneven in art and introspection, but some of them provide some inspired insights into the characters.