Hi!
Currently, Wyrmrest Accord has a one of the larger Horde populations, but Moon Guard’s is fairly healthy too 
Cenarion Circle/Sisters of Elune are smaller, but their merged server has a bigger focus on guild RP. However, you can find guild RP on Wymrest Accord and Moon Guard, as well as an easier time finding casual RP.
I suggest checking out their respective server forums, and seeing if there’s any guild that catches your fancy!
Wyrmrest Accord
Moon Guard
Sisters of Elune/Cenarion Circle
Wyrmrest Accord also maintains a list of RP guilds and events you can check out. I don’t know how up-to-date it is, but if you check at the bottom, looks like there are more recent guild ads!
Wyrmrest Accord Guild Directory
Hope it helps!