Hey!
I think Signifyd is a great choice, it has a lot of great reviews and it looks like you can also get a free version if you display their banner at the bottom of your page. If you wanted something free without displaying a banner you could check out Fraud Filter:
https://apps.shopify.com/fraud-filter
Here are some other apps with good reviews:
Riskified
https://apps.shopify.com/riskified
Subuno (Free!)
https://apps.shopify.com/subuno
Merchant Protector
https://apps.shopify.com/merchant-protector
Hope this helps!
Yasmin