Quentin's death would have made the most sense, as there is not many more places to go with the character. Laurel, in comparison, had far more potential and growing to do. There are far more possibilities for the show with Laurel on it than Quentin. His death would also have had an effect on lots of people. It would have affected Oliver, Laurel, Felicity (because of the horrible effect that it would have on her mother, Donna). And while to a much lesser extent, Diggle and Thea would also have been hurt by it, as I am sure that they have grown to appreciate and respect the former police captain more over the course of the season, as he has aided them by providing an insight into Darhk's operations and risked a lot in doing so.
It also would have fit his character perfectly. His character is a tragic one. He is a man who cannot find happiness, and when he starts to come close to it, it is swiftly and cruelly taken away from him and he is put in a worse position than he was before. What makes it worse is that he is a good man, a kind person, that loves his family more than anything and who just wants to help people and make the city a better place. Despite the flaws that he has, he deserves better. If he had to be killed off the show, there would have been no ending more fitting, more appropriate, more right than him going out protecting Laurel or Sara, just when he was beginning to find happiness in his life with Donna and his relationship with Laurel being in a very good place. It would have summed up the man and his life perfectly and would have been quite beautiful in a way, if not sad too. Tragic characters normally have tragic ends. The Lance family and their lives are filled with tragedy, but one member of the family had much more tragedy to face, obstacles to overcome and grow stringer for it, while the other should have been coming closer to their tragic end. They got them mixed up though.