I guess it depends how you define biggest but everything we (Leadent Digital) build is serverless. We have a last mile field service product called On My Way that's whitelabelled by one of the largest field service management vendors and also sold direct to the likes of Foxtel in Australia, Bosch Thermotechnology, and companies across the UK, USA and Canada.
We've also built a number of other products (all in the field service niche) used by some pretty big organisations in the UK like, Anglian Water, Northumbrian Water, E.ON Energy, Calor Gas and some local governments.