The most robust watch crystals are made out of sapphire glass. Only diamonds and carbide is harder and can scratch sapphire glass. Of course there are other materials such as some metals and stones which will scratch your sapphire glass crystal, so it is not wise to expect the watch to handle any abuse without getting marked. Sapphire has one downside though, it is highly reflective. Some watches receive an anti-glare coating to compensate for this, but sadly this coating is less hard and will scratch more easily.

Different from titan watches, which are just timepieces for very large people out of the Greek mythology, titanium watches are made from titanium. This is a very hard metal, making the watches very durable and scratch resistant. Plus this metal is surprisingly light, so titanium watches will not dislocate your shoulder every time you swing your arm. It is these to qualities that make titanium popular with space technology engineers, racing car makers and other high-end gadgetry. In addition to being just a very suitable material for making watches out of, it is also the case material of choice for people with a nickel allergy who have too much style for plastic watches. Titanium watches definitely do not contain nickel.
Titanium is also darker than stainless steel. It looks greyer and is less shiny than the silvery gleam of stainless steel watches.