I vote for IB(Interactive Brokers). I’ve used them in the past as was quite happy. Pinnacle Capital Markets trading also has an API (pcmtrading.com) but I haven’t used them.
Interactive Brokers:
https://www.interactivebrokers.com/en/?f=%2Fen%2Fsoftware%2Fibapi.php
Pinnacle Capital Markets:
http://www.pcmtrading.com/es/technology/api.html