I can't say which event because where were a lot; especially in the earlier days. Some matches though would be
SeXXXy Eddy vs. Arsenal - TOD 3 -- Eddy somehow cut an artery in his arm. Was legitimately squirting blood. You've probably seen the clip many times of him drinking it as it squirts out. He actually got lucky with that because from what I heard he was gonna be massacred in the finals.
Masada vs. Danny Havoc - New Heights 2011 -- Brutal match. I've seen a lot of crazy shit, but I've never seen anything quite like the aftermath of a Gusset Plate meeting flesh. Heard Kasai and Masada at WrestleCon was brutal as well.
I just read the review and saw still images of Masada vs Havoc 2011 and that was totally crazy! Link
I am going to check out the others suggested here now.
Reminds of when Jim Cornette said it best: "Wrestling used to be about people pretending to hurt each other and everyone thinking it was real, to people actually hurting each other and everyone thinking it's fake."