The best one in my opinion are the Shadow Caves! usually known as Ab'Dendriel Troll caves!
You need to travel to Ab'Dendriel and have 50 gold coins in order to buy a key from Shirith. Once you do that, you can access the doors leading to the caves. During Rapid Respawn, these caves are extremely easy to complete your bestiary and even during normal respawn it shall be pretty easy!
You can check on it is this place here, where I walked around in the cave until the last floor:
https://www.youtube.com/watch?v=2UkZClfkOSA