Act 6 IMO is the best. It is the ultimate culmination of the themes and concepts explored in the first 5 acts. Early act 6 with the alphas flows really well without feeling like a retread or a waste of time (which is kinda how I feel about act 6 act 1. I want to comment more indepth on the entirety of act 6 but I have only read it once and am still in the middle of my reread. The epilogues are a close second if we count them.