I'm a fan of OpenFaaS. It's a simple, smart design, and Alex has done great work in building a community around it. We found it easy to adopt and support for garden.io, and it fits very neatly into the Kubernetes ecosystem.
Alternatives inside GitLab Serverless (https://about.gitlab.com/product/serverless/) would be great. At the moment they have Knative, but enabling more gives a richer possible experience.