Using "am meisten" to elevate a participle to the superlative (as it is done in English) would be an Anglicism - the superlative of "das gefürchtete" is, indeed, "das gefürchtetste".
There is, however, a subtle twist why in your special case it isn't [an Anglicism].
"Das am meisten gefürchtete" is in fact the superlative of "das viel gefürchtete" (often contracted into one word).
So, in fact there is a very small difference in meaning between the two:
"Das am meisten gefürchtete" is more a superlative in terms of number of instances (i.e., there are a lot of people that fear the thing), while "das gefürchtetste" is a superlative more in quality (i.e. people fear the thing a lot). The difference is, however, so small, that many people would assume the two examples to be synonyms, would probably use it interchangeably, and would probably prefer to use "das am meisten ..." because "das gefürchtetste" is even twisting a native tongue quite a bit.