Most Valuable Conference Hallways

In Eric Norlin’s Digital Identity World Recap #2 (1 and 3 coming soon) the writes that

Jonathan Penn (of Forrester) declined to be on a panel because he thought he’d get more value by being in the audience (and he said I could quote him).

I have heard and read other people say that some conferences are valuable simply for the hallway conversations.

So my question:
What are the top five most valuable conferences simply due to the quality of the hallway conversations?