Zoltan of Aegwynn is the guy who's been sitting on top of the overall achievement point lists for a while, and he's just recently cracked the incredible total of 10,000 points. At the time of this writing, he's finished off a total of 908 out of 931 achievements currently in the game, giving him a total of 10,005 achievement points.
The Champion of Ulduar is the one that put him over the top according to his Armory profile,
Very impressive indeed. Zoltan has just one or two people on his heels, and then the numbers drop out to a few hundred below 10,000 (and strangely enough, the top five players are all EU folks). There's no reward for all of those points (and Blizzard has no plans to offer any), but it's quite, as you might say, an achievement.