If I understand the question right: Which was the first S-CLASS ?

The class all by itself ?
That would be the 108/109 series as their body had a totally different design and panels, than the 'lower' class models (first 110/111/112 and later 114/5).
Although the w111/112 sedans shared the same body and many other parts with the 110 or even some inferior 111 (220b, 230S...).
They are usually differentiated by "Big Fin body" or "small Fin body".....

The w111/2 Coupes and convertible were the first 2 door s-Class with a unique style and body not shared with others, but for sharing the w111 and w112 designation which makes it that much harder to make any sense of it.

I for one do NOT agree with Mercedes trying to name the pontons as the first E-class, just to establish heritage,...........