Being an avid DVD collector (I own 1400 DVD's) I thought I'd post my must haves' by genre.
Action
The Hitcher - A superb remake of a movie classic.
http://ecx.images-amazon.com/images/I/51dPh36HKbL._AA240_.jpg
Adventure
Lord of the Rings - The Two Towers
This is a great follow up to the first movie. The build up to the big battle scene is phenomenal the tension is amazing a must see but watch the fellowship of the ring first which is also great.
Animation
Wallace And Gromit - The Curse Of The Were Rabbit
The last film from Ardman Animation and a real beauty.
Biography
The Queen 2006
Helen Mirren at her very best.
Comedy
Freddy Got Fingered.
Astonishingly tasteless comedy. Watch out for the birth scene.
Crime
True Romance
Very violent but totally absorbing.
Documentary
Super Size Me
Shocking insight into McDonald's practices.
Drama
Green Street
Fantastic performances from Elijah Wood and Charlie Hunnam. The energy of the film is great.Wicked soundtrack, great script and an overall very cool film.
Family
Charlie & The Chocolate Factory
Johnny Depp is good at turning his hand to anything he applies himself to. He makes a wacky, fun Willy Wonka.
Fantasy
Premonition
Storyline is a clever idea and there are some genuinely spooky moments in this film. It is also quite moving.
Horror
Hostel
Classic Quentin Tarantino. Don't bother with Hostel II though.
Mystery
Identity
Very underrated film. Complicated and diverse plot but totally engaging.
Romance
Sleepless in Seattle
An old one but a good one. A warm and compelling love story.
Sci-Fi
The Core
Get inside the earths core. Brilliant Sci-Fi Adventure. Just don't take it too seriously.
Thriller
Number 23
Good plot, good storyline, good effects, great acting, excellent atmosphere, superb editing and great sound.
Now I know a lot of people will disagree with the above choices. I have steered clear of the obvious normal choices and listed a few of my favorite films. I've deliberately chosen some obscure titles and I'm not saying that everyone should have these in their collection but ,if your renting a DVD and stuck for chose, then it might help you make a decision and pick something you might otherwise overlook.