Kurk pretty much nailed it on the head, and I was going to say Qui Gon, but Cupcake took that one. So, let's see...
Maybe Krayt. Talking to any of these people as equals would really be quite satisfying, but someone like Krayt, Anakin, Dooku, Thrawn, Tarkin, those who would normally ignore you, would just be amazing.
I'd like to connect with people like Vader, those who are more difficult to understand and most people don't get to really connect with. Those who've suffered. Cronal, Irek Ismaren, Mother Talzin, Ventress, the other older nightsisters.
The Father of Mortis may be a good option, and maybe the Bedlam Spirits, especially if a conversation with them is SERIOUS. If I could connect or sympathize with Abeloth, I'd be willing
Nihilus. I'd like to know what he's thinking under that mask. It's probably not very INTERESTING, but hell, it would be stimulating.
Xendor, possibly people from even before him. I've always been interested in the pre-hyperspace age. I love how that's pretty much where humanity is right now, but in the Star Wars Universe, it's treated as pre-history. It'd be interesting to get to know those who figured out the secrets of the force for the first time, and by their own discovery. Rajivari, Deagen Lok, even more ancient civilizations whose origins are unknown
Much as I love Jedi, and would love to talk to some, I find the sith to be far more interesting, so Plaguies, Tenebrous, Karness Muur, Exar Kun, Treya, Sion, Ajuta Pall, Andeddu, Bane, Zannah, and Vitiate are going to be on the list
As far as scientists go, I was in love with the "galaxy of fear" book series as a kid, so Borbygymous Gog would not be an unwelcome conversation.
I've always been fascinated with the Anzat, so Dannik Jericho is probably going to make for some nice conversation.
There are more, but there are just SO many more...