There are other beers that are malt-forward, but I can't think of much of anything that's more specifically malt-focused than the dopplebock. Maybe Utopias, but even that is fairly heavily dominated by the maple syrup additions. I've only had Fat Tire a couple times, but I recall that being more of a balanced pale ale, with some malt but still reasonably balanced.
EDIT: I'd also say that it's not necessarily the "least bitter" that you're looking for, as that would probably be something like Mich Ultra. It's the lowest bittering-to-gravity ratio (BU:GU), the relationship between the beer's IBUs and its original gravity. A beer like a dopplebock may still have a decent amount of IBUs (25-30-ish), but they won't be nearly as present as if the beer's starting gravity was only 1.035.
EDIT 2: Not necessarily "malt" focused in the way that a bock is "malty," but a lot of sour beers (lambics, etc) have virtually no bitterness. Traditional lambics are brewed with aged hops that have essentially 0% alpha-acids. So again, be a little careful with the specific question you are asking.
