I think it is important to understand the information or outcome that you want to achieve by asking the question, as it will determine the relevance of the response. It is difficult to do in a simple one question response as there might be a lot of contextual factors that you want to tease out.
For example, if you just want a general measure to sort the user groups into different levels of perceived computer literacy then there are probably some standard questions based on commonly used applications and tasks that you can choose from. However, if you already know something about your user group (e.g. they are an older population) then maybe you need something more specific to what they are familiar with so you can separate them into groups (otherwise you might just get everyone falling into one category).
Also, if you are testing or working on a specific type of application, you might want to ask a question that is targeted at the tasks that are typical of those applications (maybe based on usage statistics to determine how complex or common that feature/task is).
Finally, I assume that the reason you are interested in an one item question is because you might have seen things like the NPS score that can encapsulate a lot of variables into one simple figure to report on, but this is also assuming that you either don't need to be able to work out later on what those factors are by not collecting information on it.
I don't know of any 'magical' metrics in UX because the research that we do are so highly context dependent, but if you come across any then I'd be keen to test them out as well!