I would have to say (and anyone can call me on this) but there is NOT a modern day covering that strengthens the structure as well as silk, silkspan, or jap tissue. The main thing that most of the new coverings give you is ease of covering (no dope or finish), lightness, and/or tear resistance.
So, with that being said, my favorite covering right now on bigger models is Icarex (rip stock material-used by kiters- sold by Hang-Em-High). Simple to cover with and I have never put a rip in it. But, again, you need a very strong structure under it….
Everyone will have a favorite….In fact, I still like jap for small models. Others like mylar. Some still like Polyspan or Oracover….Pick your poison!!