Which event is always triggered when a client initially connects to a virtual server configured with an HTTP profile?

HTTP_DATA is not a standard iRule event.

CLIENT_DATA is triggered when payload data arrives from the client, which occurs after the connection is accepted and can fire multiple times, whereas CLIENT_ACCEPTED fires once for the connection establishment.

HTTP_REQUEST is triggered only after the entire HTTP request headers have been received and parsed by the HTTP profile, which happens significantly later than the initial TCP connection acceptance.

CLIENT_ACCEPTED is always triggered when a client's TCP connection is successfully established with the virtual server, serving as the first event in the connection flow, even before application-layer data is processed by an HTTP profile.