It is usually difficult to be very expressive and open with angel investors in an early startup, especially since the founder needs to be constantly allaying fears and convincing the investor to invest. They often don't want to ask hard hitting questions since they're afraid of any tiny thing that might change the fickle investor's mind. (Bear in mind, startups with immense traction with oversubscribed rounds tend not to have this issue and can be a lot more picky in choosing the right investors. If you're one of these guys you're probably not on MosaicHub though :P).
However, I think some of the best questions that a founder should ask an investor are:
1. "Let me know your doubts, feel free to be completely open with me" (usually gives on-the-fence angel investors the opportunity to have a more open discussion with you, which will help you allay their fears and convince them their doubts are misplaced. This is the biggest issue I have with VCs, they don't take the time to have a debate/discussion like angels do. It is either a yes and move straight to due diligence or a no with some misinformed reasoning).
2. "Will you be able to open doors? (Be specific with them here, ask specifically what value they can add, i.e. introduce to X, get X to join the investment round, etc. The more specific your request the more likely they will follow through post investment).