I don't think there is a best one. It is as simple as that. For flatland specifically there are about 20 really, REALLY good flatland frames out there. So then it's all personal preference, not quality. Complete bikes are not the way to go... unless you are on a budget. Then, it's all about money.
I gave the Darwin spec list in another post and I would consider that one of the single best complete bikes a person could build up. But, it's not available as a complete bike.
I guess that's kind of it. The BEST bike is one you build up yourself using the best parts and components. You don't find the BEST stuff on complete bikes because that would be cost prohibitive. So, you see complete bikes value rated to give you a lot of bang for the buck. But, they aren't custom.... they are stock.
So, I don't have any opinion on the best. I just know when stuff isn't as good as it could be and am willing to point towards alternatives. In another post someone spoke about one of the Mosh bikes, which I didn't like that much, but there is another bike that Mosh makes that seems a lot better. So, even within a single company you can find good and bad products and need to accept that a company that makes 1,000 crappy parts can still make that ONE gem of a product that every rider should run.
Primo is a mixed bag of components - some are great, some aren't.
Odyssey, same way.
Haro has some excellent components you can get at a good price - but a lot of their stuff isn't that great... or is beaten by others.
You get the gist.