Real SMS Flatrate Provider for Braintower Gateways

Hi all,
simple question:
We are looking for a new provider for our braintower SMS gateway.
As we are most likely not the only business with a SMS gateway, I wanted to ask you guys, with which providers do you have good experiences. We just need a pure SMS Flatrate (many providers call it Flatrate and limit it to a few thousand SMS). PMs are welcomed, too.