I've been using Archidekt. I really like the built-in playtester, but the interface can be kind of sluggish, and some of the design decisions, especially around categorizing and sorting, are quite clunky.
I know there are a lot of options but I haven't tried any of them. Are there any that you'd recommend?
Moxfield, I've tried most of the others and like this one best.
A second for Moxfield here. I started in Tappedout, but found it too ugly, switched to Archidekt but got frustrated by errors like duplicate cards appearing, and finally ended up at Moxfield. Functionally it's very similar to Archidekt but I haven't noticed any bugs.