Berlinghoff Rasmussen. His initial time traveling thief gimmick was only good for one episode, but rather than drop him back off in his own time like many time travel episodes often do with pests from the past, they kept him as a prisoner in the 24th century. It would have been interesting to see him making it as a criminal on his own after getting out, only to have the TNG crew show up and need to use his connections for something, or have him try to swindle Quark on DS9.