Illegal Alien is the proper term.
I prefer "Criminal alien", since it means the same thing (grammatically), but, the modifier has NOT been taken over by the propagandists. The word "illegal", in this context, has become like the word "gay" or "queer" -- divorced from its original meaning, and imbued with a meaning of the propagandist's choice, bearing no semblance to the original.
So, I call them "Criminal aliens" -- and I challenge the propagandists to try screwing with THIS nomenclature! Let's see them try lecturing us on how we treat "law-abiding criminal aliens"!