It all depends on the BD. Usually the largest file size is the main title. Some disks have the movie with multiple camera angles so you pick the one you want, some disks have just one title, and some will have hundreds of titles (with only
one being the correct playlist/title) called "playlist obfuscation". Then you may have to deal with Cinavia on some disks (which can be an issue if you try to play the rip on a licensed player. Ripping BDs is not that easy.
If your asking about which software ripping products, there are three "big ones".
MakeMKV is currently free and removes the copy protection, but your on your own figuring out what title to rip. The makeMKV forum will often have a post when a new BD comes out identifying the correct title (playlist) to rip. I don't think makeMKV can remove Cinavia from the rip.
http://www.makemkv.com/There is also DVDFab which also removes copy protection and Cinavia. (I don't use this software so I can't comment how well it works).
http://www.dvdfab.cn/blu-ray-ripper.htmThen there is the AnyDVD HD + CloneBD solution. As you probably know AnyDVD HD removes the copy protection from BDs and it has Cinavia detection. If you use CloneBD (from Slysoft) it will rip a BD to mkv or mp4, can shrink the movie if desired (say to copy to BD25 disk), remove the extras, will identify the main movie, and has a little "preview" screen where you watch the various titles on a disk to see what they are. But even then, sometimes when a new BD disk comes out with multiple titles, you still need to go the CloneBD (or AnyDVD) forum to see what is the correct title.
http://www.slysoft.com/en/clonebd.html