[Dismas]

Normally for packet injection I recommend anything with an RTL8187 chipset, which are used in many adapters, namely the Alfa AWUS036H which is still one of my favorite adapters considering the power for the price. The only problem would be the Raspberry Pi being a low power device, you would probably need to use a powered USB hub with it. I have a small Ralink RT5370 wifi adapter for my Raspberry Pi that works well, is packet injection capable, and doesn't need a powered hub but also has low range, if you're looking to sniff networks over any large distance it wont work but for general wifi usage and close range wireless attacks it works well.

Exact same adapter I would recommend. I have 2-3.