==> Couldn't you just say, "The brownies on the right are the most moist" and avoid moister/moistest altogether?
It depends on whether or not there are two groups of brownies or three or more groups. If there are two groups, then you use the comparative form - "The brownies on the right are the more moist". If there are three or more groups, then you use the superlative - "The brownies on the right are the most moist".
It doesn't matter whether the comparative is in the form of "more <quality>" or in the form of <quality>-er, you use the comparative for two items. Similarly, it doesn't matter if the superlative is in the form of "most <quality>" or "<quality>-est", the superlative should not be used unless there are three or more items involved.
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