What’s the best "Joining Sect Arc" you’ve seen. It can be trials, the sect atmosphere or even how it works.
When Meng Hao from I Shall Seal the Heavens joins the Violet Fate sect. The reason it`s one of my favorite "joining the sect" arcs is that despite Meng Hao having earlier grievances with that sect, instead of the typical "I join them and destroy them from the inside once they find out who I am", he actually becomes one of their top disciples with his fake persona and is even made a legacy disciple despite someone of the top brass discovering who he really is. It`s also probably the funnest arc in the series.