Wow. I don't know if it's the greatest, but it's big (check), unbelievably detailed (check), has lots of trains (check) and shows that a lot of hard work was put into making this an eye-popping layout. It's definitely one of the best-looking layouts - comparable in workmanship and in attention to detail to a local club layout in Montreal that I've visited on several occasions during their open house days every November. It's enough to make me camp out under the benchwork at closing time...(check) .
There's also Northlandz in Flemington, NJ which claims to be the world's largest model railroad at 52,000 SqFt in HO. Despite having been in Flemington hundreds of times, I've never seen it. It costs $36 per person and large doesn't always equate to great. Maybe someday.
That's Big with a capital B. I've watched a couple of videos of that one some time ago, and a few webpages on it. It's crazy awesome. One can walk around, crawl under, pass over and generally wander around the huge maze for hours. The sheer size is incomprehensible. If I would be 10 years old again, and visit that place, I'd never want to come out. They'd need sniffer dogs to find me!