Best dialogue options during citadel departure speech?

Generally speaking, the upper left hand option is the option for Paragon points, and the lower left hand option is the option for Renegade points. Choosing the middle left option generally gives you no change in points either way.

The easiest way to check what a particular dialog option is worth is using this Mass Effect Wikia entry. At the Normandy speech, you have 2 chances to choose Paragon or Renegade responses, worth 2 points each:

Paragon:

• "Humanity must do its part."
• "Everyone is counting on us."

Renegade:

• "Nobody's going to help us."
• "Humanity's in this alone."

Here's a video of both speeches, back to back, choosing the Paragon options first, and the Renegade options second: