I recently posted a list of Criterion releases I consider "essential". Without knowing your taste in films (and even then ...), it's impossible to suggest blind buys that you're guaranteed to enjoy.
As for the three discs you mentioned (all of which appear on my list), they are among my favorites. The Passion of Joan of Arc is profoundly moving and a revelation. The commentary on Children of Paradise (especially Brian Stonehill's on disc 1) is brilliant. 8½ receives the royal 2-disc treatment it deserves (although I find the film on disc 2, Fellini: A Director's Notebook to be self-indulgent & pretty worthless).
As for the three discs you mentioned (all of which appear on my list), they are among my favorites. The Passion of Joan of Arc is profoundly moving and a revelation. The commentary on Children of Paradise (especially Brian Stonehill's on disc 1) is brilliant. 8½ receives the royal 2-disc treatment it deserves (although I find the film on disc 2, Fellini: A Director's Notebook to be self-indulgent & pretty worthless).