Most Challenging Chants

I'm teaching an advanced voice student Gregorian notation and wish to challenge him with an elaborate selection from the Gregorian repertoire. Unfortunately, my limited knowledge of merely the OF Introits and Communios only extends to works of moderate difficulty. Google has been unhelpful thus far. I'm looking for something florid and challenging that would ideally display a wide variety of neumes and neumatic combinations for pedagogical purposes as well.

Anything off the top of the community's head that might fit the bill?