Jaqen H'ghar.
Eulalia, it was Irri. And I'm glad you chose Irri and not Renly. It is known.
Littlefinger has poisoned at least two people (through intermediaries, of course), but I would not want him anywhere near my dinner.
Also, I'd not want Walder Frey hosting the dinner.
At first glance, this thread looks fairly light hearted... but who you choose depends upon what you want to get back. For myself, I'd like information.
Specifically, I'd like to know...
What was the nature of Rhaegar's relationship with Lyanna?
Who are Jon Snow's parents?
Is Dany barren? (I think this goes to the heart of TPTWP.)
Did Aerys have any extramarital relations? Did he rape anyone besides his wife?
What are the connections between The Prince That Was Promised and Azor Ahai Reborn?
What was the sequence of interpretations of TPTWP that Aemon and Rhaegar went through?
The people who hold the most information regarding these critical events leading up to and during Robert's Rebellion are Maester Aemon, Varys, Howland Reed, Eddard Stark, Maester Pycelle, Barristan Selmy, Jon Connington, Jaime Lannister, and Ashara Dayne. I know that's more than three people, so I'll narrow it down.
Ashara is supposedly dead. But I suspect her knowledge can be covered by Varys, Connington, and Selmy. Mainly Connington... I suspect she and he were involved to get Aegon out of KL. She may have faked her death to join Connington in raising Aegon.
Connington may be dead... or not. I also suppose that what he knows is also known by Varys.
Jaime was not as close to Rhaegar as Barristan, Ashara or Connington. He's already spilled most, if not all, of his secrets to Brienne.
Eddard... Ned's dead, baby. Ned's dead.
Barristan knows what Jaime knows. He probably knows or could make excellent guesses as to the knowledge that Ashara and Connington possess.
Pycelle knows alot, but Varys knows everything Pycelle knows plus a LOT more.
Varys... the mysteries and enigmas of Aegon, Lyanna, Rhaegar, Aerys II, plus many more are within his brain.
Aemon knows Targaryen family history intimately. He was also in regular correspondence with Rhaegar concering TPTWP. Of course we now know his dying thought was Dany=TPTWP, but I'd really like to know what Rhaegar thought regarding TPTWP.
Howland Reed knows THE secret. But I suspect that if a person could access the secrets of Varys, Aemon, and Barristan, then I think a person could accurately guess Reed's secret.
So my answer is Varys (for the secrets of Aegon, Aerys, and R+L), Aemon for the secrets of R+L and TPTWP), and Barristan (for the secrets of Aerys and Rhaegar).
Edit: Pyar, I forgot to say that your post cracked me up.