Good V1 is a great choice for Sony a6400.

I recommend the Godox V1 or Godox V860 III.

The new released Godox V1 Pro is even better.

Here are some top recommendations:

1. Sony HVL-F32M

• Compact and lightweight flash designed for Sony's mirrorless cameras
• Guide number of 32 (at ISO 100)
• Tiltable flash head and bounce capabilities
• Wireless control with optical transmission
• Good balance of power, features and price

2. Godox TT350S

• Compact flash with a guide number of 36 (at ISO 100)
• Full TTL support for Sony cameras
• 360° rotation and tilt-able flash head
• 2.4GHz wireless radio transmission
• Affordable third-party option with good performance

3. Sony HVL-F45RM

• Mid-range flash with a guide number of 45 (at ISO 100)
• Radio controlled wireless shooting up to 30m
• Tiltable and rotatable flash head with bounce and swivel
• Quick shift bounce system and modeling light
• Higher power and more advanced features compared to the F32M

Like other said Godox V1 and V860iii are best choices for Sony a6400.