Wait, what? I just voted Reach, but it says that one person has voted "None: Save your money."
If I accidentally voted for that, disregard it. I love the Halo series. But I would certainly not recommend starting with Halo 3: never, EVER start a series with anything but the first one.
However, between your poll options, Reach, being a prequel, may give a bit more context to the events of the first game. And despite what people say, I really liked the story of the Halo series. Meaning that you can start with Reach if you want (it's looking amazing, and the multiplayer beta is fantastic

), but do not play any of the trilogy out of order: the order I recommend is this:
Halo
Halo 2
Halo 3
Halo 3: ODST
Halo: Reach
But as said, you can play Reach first. That would be fine.