Diehard batteries used to be made by Interstate. Now, they are made by several different companies, depending on the model line. Diehard Platinum is produced by Enersys, who makes the Odyssey line of batteries. I've had a Diehard Platinum Group 35 in my FJ for a year now with no issues. It's rated at 850CCA. When I purchased it, I tested it with my Newmar Digital Battery Analyzer. It tested at over 1200CCA, impressive to me. Batteries always test about 20% higher when new, but this was obviously much higher than that.
There's also the Duracell AGM line at Sam's Club. It's a re-badged Deka Intimidator AGM, which is made by East-Penn. These have the best "Bang For Your Buck" of any AGM Group 31 I've ever experienced. I believe they are $189/each. Put 4 of the Group 31's in a customer's boat this spring. Didn't test them, but one of them powered two 200a thrusters without issue. No scientific stuff here, but in a couple years of experience with these thrusters, I was impressed with how well they handled the load and took a good amount of time berfore they faded.
There's also a fairly new company called Northstar Batteries. From what I understand, it's a couple guys that broke away from Enersys or Odyssey, which is why they use the same housing(purchasing from the same source). Don't know much about them yet, but from what I've hear, they are similar the Odyssey
Anyhow, just my .02 Cents. YMMV