How to choose a flyback diode for a relay?
First determine the coil current when the coil is on. This is the current that will flow through the diode when the coil is switched off. In your relay, the coil current is shown as 79.4 mA. Specify a diode for at least 79.4 mA current. In your case, a 1N4001 current rating far exceeds the requirement.
The diode reverse voltage rating should be at least the voltage applied to the relay coil. Normally a designer puts in plenty of reserve in the reverse rating. A diode in your application having 50 volts would be more than adequate. Again 1N4001 will do the job.
Additionally, the 1N4007 (in single purchase quantities) costs the same but has 1000 volt rating.
Voltage required is the nominal coil voltage, since that is what will be applied. Give it a factor of 2 for safety.
Current requirement is the nominal coil current.
Speed is probably not a consideration for relay coils, since they are not turned on/off very often, as compared to, for instance, a PWM motor drive.
In your case, a 1N4001 will probably work just fine.
Things aren't always as simple as they seem, though in the case of relays it's highly application dependent. While the diode provides a safe discharge path that preserves your switching transistor and power supply, it can cause a few issues in certain applications. Relays on closure can form a small weld at the contacts, and by placing the diode there you are essentially preventing the relay from opening with its full force. This can cause the contacts to 'stick' together slightly longer, and overall is bad for the relay.
A trick I learned a few years ago to prevent that from happening was to put a zener diode in series (obviously in different direction) with the the regular diode, this allows you to control the maximum voltage and allows the relay's coil to discharge in a slightly better way.
Page 13 of this Panasonic application guide contains the zener diode and other options for contact protection: https://www.panasonic-electric-works.com/pew/eu/downloads/technical_information_relay_en.pdf