what RC radio transmitter to use?

To control any RC vehicle, you need a radio transmitter to send control commands to the model.
If you have more than one model, or want to buy model and transmitter separately, you need to ensure that radio protocols of your receiver and your transmitter are matching.
If you keep buying RTFs, you end up with a pile of cheap controllers that are all incompatible to each other.
So you are serious about this hobby, you need *one* good transmitter and a strategy!

The most common frequency for radio control links is 2.4GHz. All modern RC protocols on 2.4GHz use some sort of signal spreading system, which lets the signal hop across the available bandwidth and look for "free air space" automatically, and allows the connection to co-exist with other RF activity (e.g. other FPV pilots) on the same 2.4GHz band.

The three most popular radio protocols for micro quads are DSM, FrSky and FlySky.
All three protocols are basically doing the same thing, telling your copter where to fly. But they are not compatible to each other. A DSM transmitter will not work with a FrSky of FlySky receiver.

DSM is a protocol used first and primarily by Spektrum, a US based company that makes a large range of very good quality and very easy to use radio transmitters. The main disadvantage of Spektrum radios is the high price.

FrSky is a protocol used by a Chinese company producing "Taranis" radios. These transmitter run the open source operating system OpenTX, which means that even the entry level hardware supports advanced features. You are not paying for the features, just for the hardware. And thanks to an avid community, the features in a $150 OpenTX radio go beyond what you can do on the flagship Spektrum. But you need to learn a few things and do some complicated stuff yourself.
You can get spare parts and upgrade parts (e.g. hall sensor gimbals) for Taranis radios.

FlySky makes cheap transmitters. They are better than most stock transmitters that you get in a RTF box, but they are significanly behind Spektrum and Taranis in hardware quality and features.

When you understand the control link and protocol, make yourself familiar with 5.8GHz video transmission! Read on..