Whatever reference you use, this will definitely be screened as not level. My experience tells me that the sign will not be seen as a reliable reference - it is not very tall and so you can give the photo quite a large rotation without this looking too far wrong.
I tried to level the photo using the line of the edge of the taxiway, behind the sign. That way the photo looks more level, and the vegetation at the back of the photo also looks better. The sign then looked very slightly off to me, but this was giving it almost 3 degrees of rotation - a big number for this kind of issue. So I think you can afford to rotate it by something like that and generally the whole thing will look a lot less like it needs CCW rotation. For me, as it stands now, it definitely needs some of this.
Hope this helps.