IBM Watson API
Offers a variety of services for building cognitive apps.
Top 2018 Machine learning APIs
Intellexer
Processing (NLP) Service including sentiment analysis, content extraction, and language detection
unplugg
Forecasting API for timeseries data
cloudmersive
Image captioning, face recognition, NSFW classification
keen io
Data Analytics
wit.ai
Natural Language Processing
clarifai
Computer Vision
dialogflow
Natural Language Processing