I would personally love to see Temple of Elemental Evil, but I can't fathom how you could fit that into 2-3 hours without making it suck.
If they were willing to commit to a trilogy, I think the War of the Lance would be great, but they already tried an animated version that bombed.
Honestly, I think the simplest and best way to create a D&D movie would be to base it out of the Realms, in one of the better known areas (Waterdeep, Baldur's Gate, the Dalelands, Hillsfar). Create brand new characters, using the most common stereotypes (elf archer, human wizard, dwarf cleric, halfling rogue, half-orc barbarian, maybe a half-elf bard to round it out). Design the storyline to be a simple, one-shot adventure for low level characters, with a memorable BBEG monster. Like a D&D adventure, the story should have a conclusion, but with hints of something sinister behind the initial BBEG (in case a sequel is desired). A cut down version of the Starter Set adventure would work, but I think it'd be better for something new (not only so veterans don't know the outcome, but also to sell the adventure later).