By "event" I mean crossovers, not just important stories confined to one title.

I would say the best for me are Knightfall and No Man's Land, for sentimental reasons (though NML fumbles right at the ten yard line with a wonky story just to give Joker the final boss role). Knightfall made me love the comics version. I also enjoy Officer Down a lot, if that counts.

Worst are War Games and War Crimes, and it isn't even close. That thing would not end! I also dislike Murderer and Fugitive for much the same reasons, it went on and on and could've been told entirely in three months in a couple titles, in and out.