Gold is by far the softest. The term carat refers to how many 24ths gold it contains; so, 14 carat is 14, 24ths gold. This is harder than the more valuable 18, 24ths gold.
I had some trouble finding hardnesses for titanium and stainless on the same scale which makes me suspect that titanium is softer.
As was suggested by another post, the alloy matters a great deal. Commercially pure titanium should be relative soft, whereas the widely used aerospace alloy, Ti-6Al-4V should be harder. Because the hardnesses I have seen are on the Rockwell C scale, my guess is that they are less hard than stainless.
Finally, stainless steels are divided into a groups with austenic and martensitic being common ones. Austenitic stainless steels have great corrosion resistance but are relatively soft. Common stainlesses such as 304 or 316 are austenitic. Occasionally you will hear someone say that you can't get a stainless knife to hold an edge. People who think that are thinking of austenitic stainlesses. On the other hand, martensitic stainlesses can be extremely hard. This in turn makes them hold an edge in the case of knives. I don't know whether anyone makes a watch case out of martensitic stainlesses like 440C, but my sense is that it might be a good choice.
In any case, to say that stainless is soft and scratches easily is not the whole story. I have stainless watch bands that are badly scratched and others that seem to be highly scratch resistant. There is really no reason why anyone needs to accept scratches because the manufacturer could have avoided the problem with better alloy selection.
I have seen much made of the matte vs polish finish. Interetingly my least scratched watch band has regions of matte and regions of high polish. Both are equally scratch free.
