I work on an Esri heavy team, i'm looking for a mapping engine that will give me flexibility to build on top of and leverage only the pieces I need and or add more as the app grows. I've been using ESRI's API to great success for my indiviual code but leveraging their codebase into a reusable solution grows too large.
We have ArcGIS Server 10.1 and create featureserver and mapserver services. Along with those I will consume data thats created from various tabular locations and sent to my front-end via JSON/Ajax.
I've started on Leaflet, and have began testing on esri-leaflet to some success, dynamic layers are drawing slowly, but we are currently upgrading to 10.1 so that may wash out.