I would level the sink and figure a way to deal with the gap. A couple of ideas come to mind which may or may not be applicable. Would it be possible to remove the wall covering (drywall?) and sister new studs that were plumb to the the existing studs then redo the wall? The other option would be to shim the gap and caulk it. The latter would be much easier, but wouldn't look as good as the former. There may be some other ways that I haven't thought of, so stay tuned.