The greatest dupe was in 2006. It pretty much fooled a lot of people (including myself) because it was a more serious subject. Also, every time anyone mentioned the word 'April Fools' the moderators came and deleted all our posts.
There was a lot of people posting, there was a lot of people complaining. This is what did it so fun and hilarious. If I recall the joke was so good it was still active for a small period of the next day.
2005 was great and so was 2006. The rest was meh, imo (did they even do anything last year?).
Here and if you're interested, there was this one too.