The vast majority of off-topic questions that we've migrated away have been to Stack Overflow. (In fact, I only see two questions migrated to a site other than SO.)
Please add Stack Overflow to the potential list of target sites when voting to close for being off-topic.
Edit: Stack Overflow continues to be, by far, where most of the questions migrated away from here go. It would be really useful to have Stack Overflow as a default migration option.
Edit 2: This trend continues. This would be really useful to have.