TD Ameritrade is the biggest brokerage we deal with that has a documented public API intended for external use, but the documentation isn't entirely accurate and the rate limits on some endpoints are fairly restrictive by default (120 req/min, which is shared across all accounts connected to an OAuth app - we had to have this bumped by several orders of magnitude for our use case). TDA has been great to work with overall though; getting in contact with the right people there was the main challenge.
Alpaca provides a great API for equities (and now crypto) trading, but the instrument and order strategy support is limited compared to the likes of TD/IBKR. They've been API-first from the start, so their API is more modern and pretty painless to work with.
Alpaca provides a great API for equities (and now crypto) trading, but the instrument and order strategy support is limited compared to the likes of TD/IBKR. They've been API-first from the start, so their API is more modern and pretty painless to work with.