I've been re-watching the Underworld films...I think they are better than their rep. The first one especially - it's story rather than action driven, it's got a great cast of actors rather than Hollywood stars (Bill Nighy, Kate Beckinsale, Martin Sheen), and it's got a satisfying plot which builds an effective mythology and comes to a satisfying finale. It's also super-stylish and the action scenes are great.
The sequels all add plenty to the story, unlike most franchises everything fits together neatly and each movie feels like a chapter of a continuing saga - not just a repeat of what came before. Rise of the Lycans gets my vote as the most effective prequel ever in terms of how well it dovetails with the original.
The only one that disappoints is Awakening, as it tries to ditch the story-driven, mythology-building approach and go for a simplistic action fest. I was glad Blood Wars got back to the original style.