Any one you want really. The Focussport and OBX are modeled after the old Cosworth and have 1 7/8" primaries merging into a 2.5" collector. The F2 has 1 3/4" primaries merging into a 2.5" collector. I have the F2 on my ST and it breathes great. You aren't really going to see gains from one over another unless your building some monster NA beast.
Buy the F2 or the Focussport. Keep it American IMO.