So I've a bit of a fascination with an occurrence in 1071. It stems largely from the brief mention of it in both Age of Empires II and in Cartoon History of the Universe Volume Three, but it is one of those key moments in history that everyone ends up ignoring and moving past. I'd go out there and say that it is the most important battle of the 11th century, but Hastings has far to much support.
And it's funny, because while Hastings established a Nation, Manzikert left nothing really worthy of Excessive Capitalization. An emperor was captured, and a treaty was signed, but both heads of state were deposed before the treaty could take effect. And even the best out there admit that Manzikert changed little.
But it's the convenient marker, the point at which we say "up until then things were like this" and "after which things became like that". Up until this point, the Byzantine Empire was the undisputed most powerful nation in the west; not as strong as it once was, but strong. After this point, it kept getting weaker. Crusades had to be called, trade was cut off, the balance of power shifted from east to west, and things changed.
My current plan is to live to at least 2072. Because life plans based on forgotten battles are genius.