Ultimately, it comes down to personal preference. There are many people with "large, wide, and flat feet", and every one of them has a different idea of what "comfortable" means to them. For example, some people like heavily insulated shoes, while others hate it. Thus, while materials and construction can change the level of insulation (or other physical quality) in a shoe, there is no way of creating a "universal" hiking boot for size 15 people.
So this isn't a question that can be answered definitively. Everyone has a different preferred fit, weight, cushioning level, and motion profile. The best advice then, is to go into a store and try out some shoes.
That said, if you don't want to go into a store, we can isolate what most people will want in a hiking boot. It comes down to three things:
Waterproofness/breathability - I really doubt anyone wants wet socks. They make your feet uncomfortable and can be a health risk. Big, wide shoes mean more room for water to get through. So you want a material that is really good at waterproofing.
However, big shoes are inherently less breathable (low surface area to volume ratio), and most waterproof materials will make it even worse. If you don't care about breathability (and many don't), then I would go for something with full-grained leather or a good trademarked waterproof synthetic; this should really keep water out. If breathability is a concern, then I would find a boot made with a high-quality quick-drying synthetic.
Durability - This applies to shoes in general, not just big ones. No one wants a shoe that rips or wears out quickly. You can't really tell how good a shoe is just by looking at it (unless you're an expert in shoemaking), or reading online reviews (which are invariably written by marketers or new owners). There are obvious clues though:
Is it made by a reputable company? A company with a brand to protect is less likely to churn out bad shoes.
Price. With shoes, you get what you pay for.
Materials. Boots made with crappy plastics are invariably bad.
Traction - No one wants to slip in mud/sand or down a slope. You want something with big lugs and a heel with a high-friction plate. Having these qualities also means having a heavier shoe, so decide accordingly.