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?
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.
I've used tappedout.net since as far as I can remember. I don't think it's better than the alternatives, I use it mostly just out of habit.
I like scryfall.com's deckbuilder too.
After trying them all, Moxfield is by far the more mature product in my mind
It's been a while since I've used it, but the site "clicked" with me quickly. I used to be an avid tappedout user
I have been using Decked Builder for about a decade.
It has its issues, but it works well for me.
