'Connexion' is the one I have come across most frequently, and most of that was when an old colleague leant me a book he got as a child about engineering feats

'Connexion' is the one I have come across most frequently, and most of that was when an old colleague leant me a book he got as a child about engineering feats from the late 1800s and early 1900s (up to about 1940). They still used the 'x' instead of 'ct' when writing it, though it was British.