This question. Should I flag it that it belongs on SO? Or should I flag it as simply off-topic? It will likely be closed there. How should I flag this sort of question?
I completely agree with slhck's answer, but I wanted to expound a bit more on a topic I feel is important.
This issue boils down to something called cross-community etiquette.
When you migrate a question to another site, you are asking volunteers to spend their time reading and then dealing with that question. If you feel that the question is not particularly high quality, it is poor cross-community etiquette to migrate the question to another site, because then, this low quality question is having "double" the amount of time invested in it, undeservingly: people on Super User are investing in it once, to read the question and determine how to flag/vote on it; and then people on the site that receives the migration will have to invest time into it again, to again determine that it's off-topic or low quality and then close or delete it.
Even worse, close votes and question deletions can only come from high rep users and/or diamond moderators, whose time and attention are at a premium. So, asking them to spend time on something that is not really worth anyone's time is a little rude.
It is important that we spend as little time as possible worrying about these questions which simply don't meet the quality criteria of any site on the StackExchange network. To ensure that, you should ask yourself, "is this a good question for ANY site?" before thinking about migration. In general, questions should be at least "slightly above average" in quality, before being considered for migration.
StackOverflow already has an existing problem where thousands of its user with some degree of moderation tools are not aware of this concept of cross-community etiquette, so they frequently migrate questions to other sites which are of very low quality, without realizing that they are taking up resources in other communities, instead of simply closing these questions and letting them rest. If other sites start migrating these bad questions from other sites to StackOverflow as well, this will only perpetuate the problem, and validate the perception that it's OK to migrate "crap" questions.
For the sake of the community, please never flag or vote to migrate questions which are low quality. You are only wasting peoples' time and detracting from the amount of volunteer time available to answer and comment on actually good questions.