How to select the right details for activating solenoid valve
The solenoid is rated for 12 volts, is rated to have a 20 ohm coil, and is rated to operate at 7.2 watts.
12V / 20ohms is 600mA, and 7.2W/12V is also 600mA.
So you need a transistor that can switch 12V voltage plus some safety margin, and can switch 600mA current plus some safety margin.
This also means that the current is so high that a regular BJT would require too much base current, so you are better off with a darlington or a FET.
Since the current is about 600mA, the flyback transistor never has to pass more than 600mA through it. So any diode that can handle 600mA short peak currents plus some margin is enough, and the DC rating may be smaller than 600mA as there never is continuous current through it.
As the coil resistance has a 7% tolerance rating, and most likely your supply voltage has some tolerance rating as well, the nominal 600mA can at worst case exceed 700mA.
If you are going to prepare for different solenoids, the same applies - simply prepare for higher voltage and higher current and it will handle any valve that does not exceed the voltage and current ratings you chose.