Which units would benefit from Desperation off the top of your head?

While I cannot provide a real list, the idea behind desperation is to generally just sort your characters by speed, and anyone you like who has a high speed (i.e. over 36) is a good choice.

Just check your heroes with the highest speed, those benefit the most.

Any fast unit. Karel Lyn Ryoma Hana Lucina Shanna Catria Peri Clair Nino Linde. It basically turns their offensive attacks into brave attacks without the -5 speed from holding a brave weapon.. And everything tends to deal more damage then a brave weapon too. I like to use it when I have specials with low cool down as well... Aka Karel

Male Corrin is definitely one to consider. If you have a SPD boon for his IVs he can achieve a very high speed stat if you put Darting Blow on him as it stacks with his Yato, and if my math is right, his maximum speed with +SPD IV, Speed +1 sacred seal and Darting Blow is 46 without any support. That allows Corrin to double most of the common threats like Ryoma and Lucina if they haven't received a speed support or Fury. If you give him Fury he'll have 43 speed plus you get better power and bulk for him and can get into Desperation range easier.
(My Male Corrin here isn't +SPD but still has 43 Speed thanks to Darting Blow and the Yato)