Saul and Roger are sure the best candidates of being Luffy's grandfather. It also could be someone else, who we don't know till now.
The question now is why Luffy wasn't so happy when Kiji mentioned his grandfather. I am not good in reading faces, but I think he got nervous back there.
So if Saul was his grandfather what would be the reasons for his reaction.:hm:
The biggest problem is, we don't know what Luffy knows or has heard about his grandfather. I personally think Luffy wouldn't mind that his grandfather was a Marine. I don't think Luffy judges people from what their job is.
Now if Luffy knows what we know about Saul, his reaction wouldn't make any sense in my eyes, cause Saul fought for his ideals and that would be something Luffy would be proud of.
But if Luffy only knows that his grandfather has betrayed the Marines, cause someone from the Marines had stated it so, then I think Luffy wouldn't be proud of his grandfather, but I don't think that this would explain his reaction.
The other possibility is Roger being Luffy's grandfather. I personally don't like this possibility but it explains his reaction in the best way for me. If Luffy finds One Piece(when would fit better I guess) and he really is a descandant of Roger than it would weaken the "fight for your dreams" motive of one piece for me. If in 20 years noone could reach it and then someone gets it who is coincidentally a descandant of Roger than it gets the touch that only someone out of this family can get it. It just gets a bit elitist(I can't put it in words like I want to). Thats the reason I don't like this possibility and when I come to think about it, this would explain Luffy's reaction the best for me. He just don't want someone to say that he reaches things cause he is a descandant of Roger. I don't find a other point for his reaction if Roger is his grandfather.(I know this argumentation is not very stable)
So I tend to say Roger is his grandfather, but I hope he isn't. It could be Saul as well but I don't get the point of Luffys reaction then and it could be someone else.
But I am sure that Oda will manage this problem in a way that it would just be fantastic.