Diamond is quite good. Didn't you get a lot of Dec from that??
I do not remember the exact values, but I earned back my initial investment ($10 USD) (Beta Starter Set), and I also made some profit. I mean profit much more than my initial investment, so it was indeed good. Nowadays the total value of my card collection is $50.63 USD, and I currently have only a small fraction of cards of what I had before.
You can always rent out your cards now, so you can earn passive Dec