Must Watch:
Shawshenk Redemption
Pulp Fuction
Schindler's List
Sin City
T2 Judgement Day
Stay away from:
Crash
The Fountain
Armageddon
Event Horizon
Matrix Revolutions
Lot of good movies on the list. It was really hard to pick those last two must see movies.
Godfather 2 is considered one of the best movies of all time and rightfully so but you really couldn't watch 2 without seeing the original.
Same with the Star Wars movies. Gotta watch em all IMO.