I like this topic. I have to be careful about the way I answer it though, because like those who've posted before me, I'm not in the place to evaluate perfect 10's as though I've drunk all tea there is to drink.
I can answer, though, based on my experience -- limited as that is. I would say that 10% or less of the teas I've had would be perfect 10's. It will be obvious to most of you how limited my experience is, so I won't belabor the point.
They are listed as follows:
2009 Select Grade TGY - Mandarin's Tea Room: This TGY made me forget about every other TGY, and most Gaoshan, I've ever had. This is a celebration tea. Like, getting married, the birth of a child, or graduating college, kind of deal. Superior. Exclusive. Magical.
Sencha Miyabi - O-Cha: I love the balance of sweet and grassy with this one.
Wuyi 3 Stamps Shui Xian - Mandarin's Tea Room: To me, this is perfect rock tea. It throws of it's liquor faster than you can pour it. Instant steeps yield very robust brews. I would drink this everyday during the winter if I could, and I will forever be searching for other teas with similar characteristics.
Maruyama Sencha - Maiko: So far, favorite asamushi, though I've only had 4 or 5 and none from the acclaimed Ippodo.
There you have it. A list subject to be updated on a weekly basis. And if you jerks buy out the stock, I shall be most irritated.
You can still vote and discuss this topic. 
I'm not one to hand out a "perfect 10" often, however, so even out of my great teas, I'd give most of them an 8 or a 9. So I said about 20% as a guess, though it may be even lower.
