I'm a dual national Belgian/Israeli and I'm entering the US with my Belgium passport and ESTA visa, do I need a visa for Israelis to leave Israel, or is showing my Israeli passport enough?
As an Israeli citizen, you don't need a visa to leave Israel. As long as you don't have a "stay of exit" order1 (צו עיכוב יציאה מהארץ) against you, you are free to leave the country.
The only thing is that upon entering or leaving Israel you are required to present your Israeli passport and not a foreign one.
1 - A.K.A. Departure Prohibition Order