After learning his true relationship with Zangetsu ... the Zangetsu that defeated Yhwach looked almost exactly like Ichigo's first Shikai. What this means is unclear, unfortunately.

The best version of the Getsuga Tensho is that of Ichigo's True Bankai, which is powerful ... that makes use of Ichigo's new dual-Zanpakut Shikai and creates an even more devastating projectile ...