By Fellini -- That would be Satyricon.  Although Satyricon is probably Fellini's most unique film (that some may brand it as un-Felliniesque), it is also his most whimsical and possibly most extreme. Runner-up: Juliet of the Spirits.

By another director/s -- my list would include Lina Wertmuller's "Seven Beauties" and "Love and Anarchy"; Pasolini's "Salo" (despite its explicitness, this is a visual Fellini feast).  I wanted to include Antonioni's "L'avventura" and Fassbinder's "In the Night of Thirteen Moons" but these would deserve another list.  What about the most Antonionian, Fassbinderian or Almodovarian film? Or maybe the most Bergmanesque? Aha, what about the most Resnaish? That would be a challenge.