Slunk, definitely, but this is one of those oddities that doesn't have the middle form, so slink and slunk, but no *slank to parallel sank. (There's also no "slunken," but "sunken" is passive voice, which doesn't make a lot of sense for "slink." Ships, yes.)

On the other hand, we have "snuck," which I gather is still considered "dialect" or "nonstandard" by many people, especially outside the U.S. It's a rare thing in modern English, a more standard verb turning into a strong verb, rather than a shift in the other direction, with "slink" taking on the more standard/regular forms.