In my opinion there are too many to pick just one. At this age it is about "process not product" the craft should involve as many of the senses as possible, develop fine motor skills while also allowing free expression. It seems like a tall order, but really if the parent or teacher just provides the supplies and steps out of the way, magic can happen. Try combining, kid-safe clay, chenille stems, scissors, paper, glue, scraps and doo-dads and see what happens. Often it is helpful to read a picture book to create a theme. Favorites creativity starting points are Leo Lionni, Eric Carle or Mem Fox. Tried and true arts and crafts are finger painting, rip and tear paper collage, fruit paint prints and play clay. My last advice is to avoid showing adult created examples and have fun!