Vault Guide or WSO Guide

In my opinion:

BIWS: These guides were simultaneously the most thorough, informative, detailed, and manageable. They do a phenomenal job of breaking up the content by difficulty, so you'll see two sections on valuation (basic and advanced), accounting, distressed/restructuring, etc. There is also some pretty comprehensive info on behaviorals; rather than telling you what to say, it tells you how to say what you should be saying, allowing you to use your own experiences and background for a compelling story. Another huge advantage is that you get future access to updated versions, so as he continually refines the material, you get the newer editions of the same guide without having to continually pay. The only downside I can see is that there is such a wealth of material that you may find it overwhelming without sufficient time to prepare; however, if you have at least a week before an interview, you can crush that whole book and be ready to breathe everlasting fire in your interviews.

WSO: Also good, but less so than BIWS. Self-admittedly (can't find the link), Patrick and team created these to be briefer than BIWS. They don't include every single technical question under the sun or every variation of behavioral, but they are a strong resource nonetheless. Knowing everything in these guides would put you in fine shape for interviews.

Vault: These are almost the most elementary resource available. I remember reading these as a first-semester sophomore in my first major course and thinking how simplistic they were. My distinct impression was that they were written for Susie Nonprofit enrolled at Tepper or Darden learning about finance for the first time so she could interview for Summer Associate roles. They are not updated frequently (every 3-5 years if I recall correctly) and have outdated information in more than a few instances. The fact that they are a free resource provided through most career centers tells the story accurately enough: they are 'table stakes' -- required reading material to get in the door, but nothing special that sets you apart from anyone else.