2 into 1 is almost always the best performer, but may not be the sound you are looking for.
There are too many combinations to list, but the more popular ones on Street Glides are usually a SE or similar aircleaner assembly and free flowing pipes like Rineharts or Vance & Hines.
No matter what combo you pick you'll need to remap your EFI, either through the dealer or by using a
Power Commander (or similar tuner).