I honestly haven't seen the 89 GF sine 1989 (aside from the oft shown highlights), so I can't really comment. 90 and 91 were both great games. The second half of 91 was pulsating stuff. Canberra clinging desperately to a 12-6 lead, Penrith throwing wave after wave at the Canberra line, before finally crakcing them to level the scores. Then Alexander nailing a long range field goal. Then Scott Gale choking big time with the short drop out, letting his whole team down who had worked so hard to keep themselves in the game. Then the fairy tale ending with Royce Simmons bagging a double and Alexander nailing it from the sideline. Penrith pulled it off through sheer effort, no doubt spurred on by their near miss 12 months earlier when they had gone very close to running the Raiders down.
Aside from 2010, 1991 is my favourite Grand Final ever.