The "best" in-app chat API can vary depending on your specific requirements and the context of your application. Here are a few popular and reputable in-app chat APIs to consider, each with its own strengths and use cases:

1. MirrorFly: MirrorFly is a real-time messaging solution provided by the company MirrorFly, which specializes in communication and messaging in Flutter & React Native technology. MirrorFly's messaging SDKs and chat APIs enable developers to integrate real-time chat, voice, and video calling features into their applications across various platforms, including web, mobile, and desktop. It is often used to build applications with messaging and communication functionalities.
2. Twilio: Twilio's Programmable Chat API allows developers to add real-time chat capabilities to their applications. It offers features like group chat, one-on-one chat, message history, and the ability to send and receive multimedia messages. Twilio is known for its reliability and scalability.
3. SendBird: SendBird provides a feature-rich chat API that can be used for various applications, including social networks, e-commerce, and online communities. It supports real-time chat, push notifications, and has SDKs for mobile and web platforms.
4. Firebase Realtime Database: Firebase, a Google-owned platform, offers a Realtime Database with real-time data synchronization. It can be used to implement real-time chat in applications and is especially popular for mobile app development. Firebase also provides Firestore, a NoSQL cloud database with similar real-time capabilities.
5. Pusher Chatkit: Pusher's Chatkit is designed for building real-time chat features. It offers pre-built UI components, presence indicators, typing indicators, and an easy-to-use API for developers.
6. PubNub: PubNub is a real-time data stream network that can be used to implement chat features in your application. It offers various SDKs and has a strong presence in the Internet of Things (IoT) space as well.
7. Zendesk Chat SDK: If you're looking for a chat solution with strong customer support and engagement features, Zendesk Chat SDK might be a good choice. It's suitable for adding live chat functionality to your customer support applications.

When choosing an in-app chat API, consider the following factors:

• Scalability: Ensure that the API can handle the number of users and messages your application will have.
• Platform Compatibility: Make sure the API supports the platforms you intend to use (e.g., web, iOS, Android).
• Features: Assess whether the API provides the features you need, such as group chat, message history, push notifications, typing indicators, and multimedia support.
• Ease of Use: Consider the ease of integration and the quality of documentation and support provided by the API provider.
• Pricing: Compare the pricing models and costs associated with using the API, especially if you expect high usage.
• Security: Ensure that the API offers adequate security features, like data encryption, user authentication, and access controls.

Ultimately, the "best" in-app chat API will depend on your project's specific needs and constraints. It's a good idea to thoroughly evaluate and even test a few options to determine which one aligns best with your application and development requirements.