SPECULATION Which Force Ghost will/would we most likely see in 8?

Excellent points on each character, @Lord Skywalker, very well thought out!
It's rather tough in a way, if they only bring one back, there has to be a good reason for it. Obviously the rumors are pointing towards Yoda, so I question what kind of advice or knowledge would he pass on, and would it be unique to him?
It could be a matter of practicality, using Yoda. He's easier compared to the Guinness/McGregor issue, and there's plenty of debate about using Christensen as Anakin again. So from my perspective, using Yoda would be the easiest and the least problematic. You avoid any backlash. (Unless they use a bad puppet or get people upset with CGI )

Story line wise, I could see why they would use any of them, it would be great to see Obi-Wan talking about normal Jedi things and the history of the Clone Wars, the Empire, and how this has lead to the First Order. It would be a great history lesson and ethics/moral class for us.
Anakin would be neat because of his perspective of being the Chose One, and what the heck that actually means. But that better be handled well or you're going to upset one or both groups of the PT haters and lovers.
Yoda would also be a great choice because of his wisdom and knowledge of the Force and Jedi ways.
Qui-Gon just doesn't make a whole lot of since because Luke never knew him, and he didn't have a personal impact in the OT.

 

Do we even know if Anakin tried to talk with Kylo? I don't understand why he wouldn't do that for his own grandson but would do that for Rey, unless being a FG he has some awesome knowledge of the future, and that explains his reasons. It also raises the questions of why would Rey still turn to the Dark Side even after Anakin (of all people) talked with her. What would he have said? He must speak about how false, wicked, and meaninglessness the Dark Side is. He can't simply give his opinion on the matter (and this is Anakin after all, when did he simply give an opinion? Haha), he would pretty much be pouring his heart out to her. Which again raises the question why he didn't do that for Ben.
If Rey does fall, there can be no redemption for her because she willingly turned to the Dark Side, she wasn't manipulated or tricked like Anakin and Ben.
@DagobahGal, as you said she would be able to identify with him, then he should be able to point out where that road takes her. Pain, suffering, loss, anger, rage, you name it, he experienced it. She can't simply say "Oh what would he know?", she's talking with the ghost of the man formally known as Darth Vader. Haha. It would have be more than the being able to relate and identify, it would be a complete rejection of the Light Side for some selfish and wicked reason. And I do not mean saving her friends. It has to be awful, and we have to hate for it! Like the way a lot of people hate Kylo after he kil... he kil... you know... the bridge scene.
But if she continues in the path of the Light, then Anakin could only prove more so that is wise and why, and a bunch of good stuff.

Now, Obi-Wan would be awesome! But I can only really see him communing with Luke. Unless he serves a greater purpose for the other characters, it would be much more personal if he were mainly (or only) talking with Luke. Of course, as I said in my earlier post, the reason of whoever the FG character is has to serve a purpose, it has to be from their unique perspective on the Force and the situation. I'm afraid old Ben is too far removed from the new characters (unless Rey really is a Kenobi!) for him to fulfill that purpose.
With Yoda, he is just as distanced from the new cast like Obi-Wan, but it makes more sense because he is known for his wisdom. I do agree that he has been overused recently, but here me out: I can't the PT and Clone Wars version. He's not wise, he's not very funny (mainly in the PT), and his purpose in those films are worlds apart (pun not fully intended) from his purpose in the OT! Do I want to see that Yoda? Heck no! I don't enjoy the Yoda. That's the version I yell at. Ask my family. It's true.
But! Do I want to see the Yoda from ESB and ROTJ? You bet I do! That Yoda is wise, and humorous, but he also has an impact. That's the Yoda I can see who can transcend and appear in different stories from different people across the galaxy. That's just my hope if we get Yoda.

And I do have to bring up the argument of practicality. Frank Oz uses his voice, and it hasn't changed that much. And anyone can do the puppet. That's easy enough. With Obi-Wan, I personally would not mind seeing Ewen McGregor reprise the role, but it would look very off when the only time we've seen him as a FG is when he looks like Alec Guinness. I'd also think a lot of people would get very mad with that, and that could be part of the reason the filmmakers are not using Obi-Wan. Save him for a spinoff.
As for Christensen... well... I generally like to give people a second chance, but I can't stand his acting. I've watched several stuff, I'm an actor myself so I've really studied it... he's awful. I'm sorry, I know I'm a jerk for saying that. If Anakin could be played by an older fellow who looks like Sebastian Shaw, then I'd be all for it.

 

Quite right, that's why I wonder if Anakin ever spoke to Ben before his fall. That would add more his brokenness trying to speak with Vader in TFA, trying to find the ghost not of Anakin, but of Vader, because they have spoke... Ugh, creepy thought.

Right, Rey has no real reason to save Ben, which is why there needs to be a lot more told about both of them, and would kind of influence a FG would have. Which, strange as it sounds, I think Yoda would be better. He could just give his cool wise Yoda advice, but with Anakin there would be too much emotion and such, that it might come across as his own perspective, not that of someone who is the eternal Force. But, there needs to be a connecting story between Rey and Kylo (if he is to be redeemed), and the FG's would only encourage their journey.

Yeah, in my opinion, Christensen is pretty much the same in everything else. Which is pretty sad, because I was trying to be nice and find a way to get him back in...

 

Haha, sorry about that. He must have had some connection before, or he's just really hoping for one with Vader. A Forceback makes more sense, good point.

Yeah, Obi-Wan would have been great. I really hope he gets a spinoff soon.

Mhm. It is...

 

Haha, same here! I really hope the Han Solo movie is amazing, mainly because we'll think the actor did a great job. But Obi-Wan? SIGN ME UP!!!