X66 Supershuttle was branded from the early 90s with the Optare Deltas and then the later DAFs - I have in my mind that the Leyland Nationals were in X66 livery too but they may not have been - I couldn't find a pic of one.

I guess there was also Superliner in the early 90s and Expresslink in the late 80s (you could argue they are the 'parents' of what is now X-Lines)

As they are technically owned by Go Ahead - the Low Fell Coaches 'Cross Tyne' service pre-dated most of GNE's efforts