1. Sheikh Ibrahim Ibn Muhammad Al Wazir, from Yemen, who represented Zaidis at the 2005 International Islamic Conference, hosted by King Abdullah of the Hashemite Kingdom of Jordan.
2. Sheikh Muhammad bin Muhammad al Mansour, born 1917, Yemeni, Imam of the Zaidis according to this list of the top 50 most influential Muslims in the world (he is ranked 26th with 10 million followers)
3. Abdul Malek Al Houthi, a leader of the al Houthi insurgency in Sa'ada, Yemen
4. Ageel bin Muhammad Badr, son of the former (deposed) King Muhammad Badr of Yemen, living in United Kingdom, listed by royalist website as next in line to the throne.
Apologies if I have left anyone off.