Seymour puts up an aggressive fight against Audrey II, and dies diving into the plant with a machete in the hopes of hacking it apart from the inside. That image of Seymour fearlessly staring down Audrey II's mouth is truly a badass way to go out.
Right before Orin reaches his last breath:
Orin: What'd I ever do to you? Seymour: Nothing. It's what you did to her.
Yes, he (it?) was the villain of the movie, but c'mon, admit it...Audrey II's final number ("Mean Green Mother From Outer Space) in the 1986 musical version of Little Shop of Horrors was easily the greatest friggin' part of the entire movie. So great, the song was nominated for an Oscar. If you don't believe me, watch for yourself.
Technically, he did the same in the original ending - it's just that he failed. The fact that he decided to do it is still awesome.
It's arguably even better in the original ending—he spends the whole show being a pushover, but then he decides enough is enough and goes down fighting!
Seymour confronting Audrey II near the end of the movie. The best part has to be what happens near the end: Audrey II attempts to kill Seymour by causing rubble to fall on him. As the evil plant and its small offspring laugh, Seymour reaches out of the rubble with an electrical wire and touches one of Audrey II's vines with said wire. This causes Audrey II and its children to electrocute, before brilliantly exploding.