Cool question. I've thought of it many times before.
And it's not an easy one to answer. As a film fan, I would go with established directors who have a track record of movies I already love. This does not include Peter Jackson who jumped the shark with King Kong as far as I'm concerned and will probably never recover.
Not Michael Bay.
Who would I trust with the D&D franchise? Joss Whedon probably. Edgar Wright (Shaun of the Dead). JJ Abrams (Star Trek/Wars), Frank Darabont (Shawshank and Walking Dead), David Fincher (Seven, Fight Club).
But Hollywood is so fickle. They could easily give this to a newcomer with the right connections who has only made glitzy but popular music videos on his resume.
As much as part of me would like to see a Drizzt adaptation, i don't think that represents "classic" DnD. Classic adventuring to me would be like Gord the Rogue in Saga of Old City and Artifact of Evil. Plan it out like a trilogy with a plot arc, give it some thought ahead of time, not tagging on sequels haphazard.
A good Dragonlance trilogy would be good too, the original 3, and even Time of the Twins, are amazing books that would adapt well to heroic/romantic/epic fantasy.
No matter what they do, I don't want hardly any CGI monsters.