All of them and none of them.

What matters most is your caser. Most cases are somewhat unrealistic and no case perfectly matches what will happen in the interview.

Rather, you need to get wide exposure to all types of case (both standard and nonstandard ones) and all types of interviewers.

Now, that said, these are ones that I particularly like

1. Columbia
   1. Cases by Firm
   2. Cases ranked by difficulty of Math, Structure, AND Creativity
2. Stern
   1. Cases by interviewer vs interviewee-led
   2. Ranked by difficulty
3. Haas
   1. Cases by interviewer vs interviewee-led
   2. Ranked by difficulty
4. Darden
   1. Cases by Firm
   2. Cases ranked by difficulty across quantitative, qualitative, and overall

Now, remember, much more important than just reading a "hard" McKinsey case or having a friend case you in it, is actually how you're cased. I can make the easiest case be your worst nightmare (as can any other coach). Make sure that, if you're really trying to get pushed hard and get prepared for the toughest cases, you're looking at hiring a coach...delivery is truly key here!