Cinematography : Which Lens is your "Must Have" for filming by Keith Newell

So I have a Canon 600D and im curious as to what everyone’s opinion is on which lens is your "Must Have" lens and why? Now, this discussion is only for filming not for photography stills. So for me, the one must have lens is the Canon EF 50mm F 1.8. I have two reasons for selecting this lens. Cost and the BEAUTIFUL shallow DoF. I've seen this lens go for anywhere between from $100.00 - $120.00. It's fast at 1.8 and sharp. If you’re looking to create a shallow depth of field cinematic look at an affordable cost you can’t beat it. What lens do you prefer and why?