You don't really tell us much about your requirements. What is "best" to you? Low cost, easy to fabricate, high permeability, high flux density, high resistivity, high strength or good corrosion resistance? 12L14 or 1215 is a good low cost option & may be good enough but you say this is a "high performance" application. The lower carbon steels like 1005 may be a bit better magnetically will be harder to machine and so soft they yield easily. It is conceivable that you might want to use silicon iron for higher electrical resistivity. Or you might want to look at ferritic stainless like 430FR if you need corrosion resistance. If you need really high flux density you want 50% iron cobalt.