6. Secret Cow Level - Diablo 2
Diablo 2 might well be one of the most perfect gaming experiences I've ever had the pleasure to take part in. Not only was the gameplay so enriching and deep that it acted like a tar pit on your life and time, not only were the battles grandiose and gory in equal measure, but the game also had A BLOODY SECRET COW LEVEL in it.
Now for casual fans who experienced this absolutely bizarre secret, it might not mean much to them, but to true Diablo fans, this wasn't just a hilarious thing to discover but was one that actually felt like vindication. For you see upon Diablo 1's release, rumors began spreading of there being a secret cow level in the game that could be accessed by clicking on a dead cow (among many other methods which were all false), and this grew to such fever pitch that the devs even had to officially deny its existence... which only made people believe the rumors even more!
So when Diablo Moo - I mean, Diablo 2 - came out, fans were delighted to find that the secret cow level DID exist and soon became one of the most infamous areas of the game, with veterans of the franchise smiling ear to ear at it's inclusion.