"Must have" - maybe none of them, "very usefull if you don't have a lot of top grade ssr hime" - many of them.
My list is following:
Fire: Heph - no a-frame def debuff on ssr, so she is very usefull
Wind: Oberon as debuffer, Rami as buffer
Tunder: maybe Cyclops or Kingu, most thunder gacha SR are bad
Water: Triton, Belphegore, Venus
Light: Diana
Dark: Diana, Belzebub, Nyarly
Or you may pick a healer hime instead if you realy need her.
P.S. If you should burn all jewels/tickets for Vohu (like majority of players including me as i guess) - save a ticket for a while, it's aviable until 28th feb. You may get someone from this list from gacha.