What is the best tool to give feedback to developers?

For many other platforms you can consider HockeyApp which offers internal user feedback features. There is a cost that may be required to use HockeyApp to it's full extent and get the most out of it for your use-case

Assuming there is a dev environment that's fully releasing privately and separately from the current live production release (for the sake of internal testing), I would consider using TestFlight for iOS apps. TestFlight exists to allow internal testers to provide feedback on iOS apps

Try out Quip instead of Trello. Taking & annotating screenshots is really helpful. So I'd combine that with a checklist in Quip. You can add more notes/context about each change. I'd then go through the list with the dev team and you can prioritize together (and also delete things). Start a google hangout, share your screen and then talk through the list. Everyone can have the doc open and collaboratively make changes. Then each of the items can be worked into however the dev team works. Could just stay as a checklist in Quip, or could go into Asana, Trello, Jira...whatever they like to use.

If you want a closer look into the code, or, want to actually have the most up-to-date source for the application in question available to test locally on your machine, then you could get your devs to get on Freeter where feedback and the dev environment are both integrated; So using Trello right alongside the live application and alongside skype could essentially all happen within Freeter at once; Allowing all internal review to proceed under one hood. imo, Freeter provides the most robust solution as you're actually running the real dev environment on your own machine and seeing the application open in a Web Browser or Emulator(for mobile) to see it live on your machine as soon as updates occur (code is updated). If you aren't tech-savvy, that wouldn't really limit you with Freeter, after initial setup and config for each application you're reviewing all you need to do is understand how to pull the latest code changes, apply them using a couple commands (which Freeter exists to make easy for you), and then launch the application right on your machine to test it; Your machine can't be a Chrome book though, you'll need a decent laptop to effectively install all the tools required.

I know this one also has real time collaboration and it's for developers. Check them out

This is must-have for front-end developers. It gives you debug-friendly user sessions.