IC Markets out of Australia seem to be the largest globally. Also, it might be leverage generosity with an A+ rating. Stick with Oanda as they still are large for the USA market.
Global by region
Larger brokers for USA
It might be wise to stick with Oanda for both the current Python API and even MotiveWave platform !!FACEBOOK ACCOUNT and TWITTER. Don't worry as I don't post stupid cat videos or what I eat!