The Mazda5 uses group 35 batteries, or you can get whatever that will fit and can be fasten down with the stock bracket but make sure it has the correct terminal post orientation.
Duralast Gold. It was at the top of the list of the Miata forums so I bought one for my '09 Miata. When the weather gets cold again, I will be buying another one for the Mazda5.
A better question is:
"Why the heck is my 3-year old battery dead?"
I did a gripe post last year about Autozone: they dropped their warranty on the red batteries to something like 3 years prorated (BOTH the red autozone battery that came with the car when I bought it at 3 years old 27,000 miles AND the red Autozone battery I replaced it with a few mos later died right around 3 years) Only the Gold batteries come with a free replacement for a specific time period, and they jacked the price up on them something like $20.
After that experience, I decided never to replace any car battery with an Autozone battery unless it has warranty replacement credit.
I got an Autocraft Gold battery from Advance Auto. No problems with it through the winter. It barely fits in the box, though, as it is taller than the OEM battery. Advance Auto seems to have a perpetual sale, although I thought the price was reasonable.