I’ve heard a few namely Ivory, Mona, Mammoth, Ice Cube, Trunks, couple others…. I used Ivory for a while but switched to Mona when they started supporting following other servers. They seem to roll out features faster than Ivory. I really like the feel of both apps but I personally use Mona. I had already bought Spring before so I got a discount buying pro.
EDIT: I forgot to include phanpy! It’s a web client but I added it to my Home Screen as a web app. I really like how all the threads are displayed on there. I use this sometimes when not using Mona. Phanpy makes it easier to get through threads and boosts on my home feed