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.
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.
I have been using Decked Builder for about a decade.
It has its issues, but it works well for me.
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
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