Various types of electrical connectors are used on fuel injectors, all of them have just two contacts. and most of the newer designs are waterproofed to prevent corrosion.
Each will require the proper type of connector.

D-Jetronic

Only found on the early Bosch and Bendix hose-end type of injectors.

EV-1 Minitimer square type

The most common type, used on Bosch injectors until the late 90's.
Also found on some Denso and JECS injectors.
Note the centered tab both inside and on the outer perimeter.

Square type / offset tab

Used on a few JECS and Denso injectors.

Keihin type

EV-6 USCAR type

Found on newer Bosch injectors.

Delphi Multec2

Only on the new slim bodied Delphi injectors.

Japanese oval

Usually found on newer Denso and JECs injectors.

Another Japanese oval type

Similar to the one shown above, also used on Denso and JECS injectors. Careful examination and you can note the different side tab location from the one above.

And another Japanese oval type

Rectangular with rounded corners and has tabs on both the sides and bottom.
This type is found on JECS side-feed injectors.