Doctor Who and BSD are both classic early 90's B titles. Doctor Who has the funnest multi ball (with the clearest objectives you can have) in pinball and BSD is one of the fastest most brutal scoring games with great theme and gameplay. I love the shot layout on BSD.
RFM is still considered a B title and that is the most advanced game in pinball history (next to WOZ).
More modern B titles for me include FGY, which has my favorite toy in pinball history - Stewie's mini playfield.