Just joined today (02/21/2022) I really like my stock exhaust on my 2018 Cadillac CTS but the two Billy Boat exhaust systems really sound aggressive. Depending on how loud and aggressive you want you CTS to sound, I would go with either Billy Boat system.
Hands down the Borla touring exhaust.
Nothing else comes close.
ZERO resonance at any cruising speed. Quiet as stock at idle but get on it and it’s a super sports car