In the Los Angeles area, at least in LA County, there really is no "capital" of McMansions.
You can find them in areas far and wide, and in diverse/different-from-each-other areas, from Venice to Downey, from Arcadia to Northridge.
McMansions are hideous anyway, but the particularly outrageously hideous ones are the "Persian Palaces" you see on LA's West Side (and I think zoning laws were recently passed to further prevent those), with the wrought iron, the huge windows that show off really over-the-top chandeliers, huge classical columns...
Armenians tend to go this route too.
And then of course there are the Arcadia Chinese McMansions that look particularly awful, because not only are they out of proportion with the lot size, but many use feng shui principles, so their proportions are really odd and totally out of scale. And then of course you'll see some with the feng shui mirrors over the front door, little waterfall elements...