After reading through 1990s Batman and Detective comics, I noticed Riddler only appears just before Knightfall when injected by Bane with Venom (Batman 490), during Knightfall when holding the tv game show hostage (Detective 662) and then going after cluemaster in knight out (Detective 704-707). That's only 3 appearances during all of the 90s. Poison Ivy also only appears in the beginning of Knightfall (Batman 495), (Detective 694) and Contagion. I thought it was interesting