Always forget? Heres the answer.
FRSKY
SBUS - RX
Smart Port - TX
or
FPORT - TX (FPort combines SBUS and Smartport Telemetry into 1 single wire, however requires firmware upgrade)
Crossfire
Ch1 - RX
Ch2 - TX
Ch3 - RX (If using Bluetooth over crossfire)
Ch4 - TX (If using Bluetooth over crossfire)
Ch12 - LQ (for RSSI)
With Bluetooth, you need to set CH4 to serial first and it will automatically fill Ch 3.
You can also connect Crossfire with 3 wires, with ch1 going to SBUS or a RX pad. See this guide on connecting Crossfire via SBUS
Flysky
IBUS - RX
Spektrum DSM Receiver
If you use a Spektrum DSM or DSMX satellite receiver, unlike pretty much every other receiver on the planet, the satellite receivers need to be powered using 3.3V instead of 5V, connect Sbus to RX
Spektrum SRXL Receiver -
Some newer Spektrum receivers, e.g., the SPM4649T, support the SRXL protocol and they can be used with 5V (amazing!). Wire the Sbus to TX
Smart Audio / ICR Tramp
Port - TX