Batman V Superman Ultimate Edition is a much, much better cut of that film than the cinema version was. Also, it is now 2.5 hours. That isn't too long for a movie like this, Dark Knight was about that length, as have a few of the marvel films been too. Given how much better this is, and how many little niggling plot details it clarifies or outright eliminates, why did the original actually get released like that. I liked this film as the cinema cut, but I would say this was a significant improvement. However, for those that really didn't care for the film, this isn't a cut that will change opinions (imo).
Saw the extended cut at a friend's home theater a couple months ago and none of us really cared for it, so I am really glad I didn't see it in the theater.
I agree with you about Guardians being the best though. I would put the first Avengers movie right up there with it and I rate Deadpool slightly ahead of Civil War, but they are all great movies.