Well depending on how far back you want to go
Marvel Secret Wars-worth reading
Marvel Secret Wars II - Not worth reading
Marvel Contest of Champions- Good Story/Bad Math
Camalot 3000- worth reading as the first ever 12 issue Maxi-series
Squadron Supreme-the original 12 issue series is worth reading
Ok all those and a few more are from back before they were called Event comics. Books like The Dark Knight Returns and Green Arrow the Long Bow Hunters are not events per se but need to be read.