Watson (V1)
Dialogflow ES (V1 in v6.4 and earlier, V2 in 7.4 and later)
Microsoft Azure Bot Service
Custom (via Orchestration server)
SM Internal / Voice Packages
Microsoft
AWS Polly
SoundHound
No support for custom TTS
Wave Folder (not part of public API)
IBM Watson
Microsoft
In development: AWS Transcribe
No support for custom STT
Local SM Speech Recognizer (not part of public API)
Deep Speech Recognizer (not part of the public API)
Local Secondary SM Speech Recognizer (not part of the public API)