Naihanchi Shodan or Gekisai Dai Ni or Pinan Nidan. Take your pick. All are easy to learn, but have sufficient depth in them to represent years and years of study. The purists will scoff at comparing the latter two to a classical form like Naihanchi #1, but I stray more and more from orthodoxy these days.
If you are prepared to teach a more methodical form, perhaps Seiunchin or a few rows of Tan Tui would be interesting. (Goju purists would say Sanchin.)
And I've always wonder what my karate would look like if I had only studied a single form like Rohai or Chinto.
I would say any of the Wado Ryu Pinan series not just Nidan.
I think you could quite happily just stick to them without going 'higher'.