I think small bunnies are easier to rehome than medium or big bunnies, although if a giant comes into a rescue, they usually get rehomed fast, because they are rare in rescues. Young bunnies or babies seem to go faster than older bunnies, lops are more popular than uppy eared ones, and bunnies with more than one colour seem to be more attractive.
Single bunnies are usually easier to rehome than pairs or bigger groups.
We have had a few REWs at ARC this year, including Luna's sisters, and most of them found a home quite fast. There are some other bunnies which you'd think would be popular, but have been with us for a while now. Most of them are medium to big sized or pairs.
I used to prefer uppy eared bunnies, because it's fun to watch their ears move, but now I have 3 lops and two uppy eared bunnies, and they all have different colours.