xcode8 list of simulators not showing

Deleting derived data and restarting Xcode worked in my case.

rm -rf ~/Library/Developer/Xcode/DerivedData/*

Ensure your Deployment Target version in Build Settings is also set to the desired version (and that you have that version of the respective simulator installed). In the below example, if that were set to 10.3 and I didn't have an iOS 10.3 simulator installed, I wouldn't be able to see any simulators. But because it's set to 10.1, and I have 10.1 simulators installed, I can see them.

If this issue is project - specific, you opened a project which has "Supported platforms" build setting set to iphone or such. Most likely this is due to simulator not having the required capabilities or an embedded library not compiled to run on simulator.

Change this setting to "iOS" to see the simulators available for running.

Set proper deployment target. And also the simulators with greater/same deployment target versions needs to be downloaded.

TL;DR sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer

A year past OP, after installing Xcode 9 beta 6 the 10.3 simulators disappeared from xcode 8. rm -rf ~/Library/Developer/Xcode/DerivedData/* didn't work this time.

Runnin xcode-select --print-path printed /Applications/Xcode.app/Contents/Developer.

Just to make sure, tried sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer, which worked after xcode restart.