0xdeadbeef52aa79d383fd61266eaa68609b39038e (beginning with deadbeef), or one with lots of 0s at the end, or some other address the user thinks looks cool.
However, because of the way the Profanity tool generated addresses, researchers discovered that it was fairly easy to reverse the brute force method used to find the keys, allowing hackers to discover the private key for a wallet created with this method.
Attackers have already been exploiting the vulnerability, with one emptying $3.3 million from various vanity addresses. 1inch wrote in their blog post that "It's not a simple task, but at this point it looks like tens of millions of dollars in cryptocurrency could be stolen, if not hundreds of millions."
The maintainer of the Profanity tool removed the code from Github as a result of the vulnerability. Someone had raised a concern about the potential for such an exploit in January, but it had gone unaddressed as the tool was not being actively maintained.