One’s best bet = the best thing one can do to achieve the results one wants;  the action or idea that is most likely to be successful.

• At this hour, taking the highway is your best bet if you want to get there on time.
• The pilot’s best bet was to make an emergency landing.
• This method of testing is the best bet for getting an unconditional result.

When the chips are down = when you are in a difficult situation, especially one which tests whether you can trust people.

• One day when the chips are down, you will know who your true friends are.
• I knew you would come and help when the chips are down.
• When the chips are down, you won’t get any help from her.

A fly on the wall = An unnoticed observer of a particular situation.  You mean you would like to hear what will be said or see what will happen while not being noticed.

• I would love to be a fly on the wall at some of these meetings.
• A fly-on-the-wall documentary about the queen’s life.
• I would love to be a fly on the wall when those two get home.