Definitely has to be Death of the Family, I remember the Wednesday that DotF's Batman #17 came out, I bought a digital copy (my first ever digital comic, because I usually get floppies). My friend (who reads my comics) and I spent a good few weeks debating and arguing how that ending came out.
While we read #17, when we hit THAT page with the big reveal, we flipped.
Needless to say, I voted for Battle of the Atom (lol jokes, obviously DotF).
AvX. It actually mattered in terms of team shake ups and created a new direction for a host of characters. Battle of the Atom has been forgotten already.
This is a good point, and it is one of the things I actually did like about AvsX was the way that it changed the directions for a lot of characters. And it was just a better story, even if I wasn't crazy about the execution of it.
BotA, was, in my opinion, just not a great story (or well executed).
That said, I do feel like part of the direction they wanted to go coming out of it was to make the individual books more autonomous, and, at least in that they seem to be going in the right direction.
It feels like BotA had really, really little impact overall. A bunch of new characters die, not that many new characters are introduced which arent some by-product of an old character, Kitty Pryde changes sides. No one really dies, no significant new X-Men, and the dream is exactly the in same position as it was before.
AvX, while bad, at least made some progress in pushing X-Books in a new direction. Just use a Before/After chart.
I say it's always a joy to read Teen Jean Grey and Teen Cyclops. Hell, the O5 travelling to the present turned out a lot better than I thought. (Then again, this is coming from a guy who surprisingly enjoys Superior Spider-Man and put's it in the top of his pile bi-weekly).
Except Warren, who has been dull and angsty and has been consistently mind-wiped since he got to the present. Also, there's something about WatX's Angel what annoys me. Maybe it's because he's too campy.