空前絶後

くうぜん　ぜつご
kuuzen zetsugo

Do you guys remember 前代未聞? It means completely unprecedented, an event or occurrence that takes things to a new level... although with 前代未聞, it's usually a new level of SUCK. I seem to remember that despite the note that it carried a negative connotation, I got an example sentence about Barack Obama becoming president, although I didn't get the sense that the author was particularly against the idea. Today's yo-ji could be described as the positive version of 前代未聞, but be careful, it also carries it's own special connotation.

Definition:
これまでにも一度も経験がなく、今後も絶対にありえないと思われるような珍しくて貴重なこと。
Translation (today Rikai Chan does it best):
1. So marvelous (or horrible) that it may be the first of it's kind, and probably the last

'Horrible' is parenthetical here. The usages of this yo-ji are mostly positive, though it does suggest a one time event. If you were to describe an Obama presidency with 空前絶後, you'd be implying that you didn't expect a repeat.

例文： マッハGoGoGoの実写版は空前絶後の映画です。CGIは本当にカッコいいけど、きちんと話の内容がなくて、残念です。
The live action Speed Racer is a movie like no-other. The effects are really cool. It's just a shame there's no substance to it.

Not that the original anime could be accused of having substance either...