I'm thinking Robots of Sherwood. I'm worried it will end up being a ridiculously silly farce, but then a lot of other people don't seem to have very high expectations for this episode either so perhaps it will just be dismissed as a light hearted bit of fluff (or maybe it will actually turn out to be very good).
So, if we're talking "punchbags", I think it will be an episode which sounded great on paper but which ended up disappointing a lot of people. "Listen", "Kill the Moon" or "In the Forest of the Night" could fit this bill.
Of course, we won't be able to know for sure until the episodes air (unless you've read the script for "Listen" of course)
Then of course (as has already been mentioned), there will always be those who complain about every episode Moffat has written so I expect most of those to attract a lot of criticism from certain people.
And finally, you'll have the odd number of people who hate every single episode and blame Moffat even if he didn't write the episode at all.