/r/IamA is a default subredddit on reddit.com with over 13,000,000 subscribers. It's one of the most popular subreedits on Reddit.com. "I am a __________ ask me anything" is the idea. This is by far one of the most popular subreddits.
The highest rate AMA ever done was:
I am Barack Obama, President of the United States -- AMA
done four years ago with almost 15,000 upvotes, and over 23,000 comments.
The next highest rated AMA was:
I am Gordon Ramsay. AMA.
this one done last year had over 10,000 upvotes and over 11,000 comments.
The third highest AMA ever done was:
I’m Apple Co-founder Steve Wozniak, Ask Me Anything!
done earlier this year with over 10,000 upvotes and over 7,000 comments.
The most infamous AMA ever done was:
I am the guy with two penises. AMA.
(yes they both work in every regard)
One of the funniest AMAs was:
I am a guy who used to make $300 per week selling my used underwear online. AMA
One guy sharing his story caused the price of used socks on Ebay to plummet faster than the great Tulip bubble burst in the 18th century.
The biggest train wreck of an the IamA subreddit was the one done by actor Woody Harrelson. While it's the norm for celebs to be promoting a movie, book, or product - most Redditors felt used when he refused to discuss anything but his upcoming moving 'Rampart'. He was vilified for turning almost every question into being about Rampart. He was made a laughing stock and a meme was spawned that continues to this day whenever someone tries to talk about a subject others will say "let's talk about Rampart" Some felt very sorry for him and believed he'd been mislead by his manager into thinking this was just a stop on his promotional tour and was not at all familiar with Reddit, or AMA's at all.
One of the biggest events on Reddit was when Victoria (known as /u/chooter on Reddit), the staff AMA coordinator who helped set up the AMAs, and often met with the celebs or special people to help them do their AMAs - was fired with no warning or explanation. The mods of IamA were so upset they set their sub private. (Nobody could enter or read there). In an act of support most of the huge default subs also "went dark" - setting their subs private for 24 hours as well. Thousands of the smaller subs also 'went dark' - shutting down their subs for the 24 hour period. The acting CEO of Reddit, Ellen Pao was vilified and all her posts were brigaded with downvotes and Redditors started referring to her as Chairman Pao, flooding /r/All with mentions of Chairman Pao and swastikas. The founder of Reddit known as /u/kn0thing said in the midst of all this "munching popcorn" - this was seen as so insensitive that it very quickly became the most downvoted comment in Reddit history. Ellen Pao resigned was made the scape goat and resigned (many believe forced to resign) over this entire situation.
Reddit admins made promises to be more transparent at that time, and have continued to make promises they do not keep on a monthly basis.
IAmA recovered, though it'll never be the same as its glory days with Victoria at the helm.
There have been a number of other subreddits spawned from IAmA including /r/casualAMA for those who are just Reddit famous, or Internet famous. As well as AMAs done in the science, sports and history subreddits. There's also a sub called /r/ShittyAMA which is rather funny :)
I recommend checking out /r/IAmA and sorting by TOP of all time for some of the best AMAs done on Reddit.