Yes, should have been more clear - there is no 'aristocracy' in Germany anymore. Still, you cannot choose such a title as part of your new name. If at all you manage to convince the authorities to allow a name-change. You do a need a psychological reference that you suffer from your current one...
The name 'Hitler': It is frequently spread around that the name 'Hitler' is not allowed in Germany. AFAIK, at least the US authorities after the war 'persuaded' all Germans with that name to better change it. And Hitler's family for example did it (his brother lifed in Hamburg until ~1950). Hitler's sister in Austria kept the name, btw. She was a pretty simple mind, and talked pretty nicely about her brother until she died...
Today, some Hitlers do life in Germany. You won't find them in telephone books, of course. Still, you won't be allowed to change your name into 'Hitler'.