Shisui's Hype

ShounenSuki:
"Heh, actually that's a pretty difficult question when you look deeper into it. At first glance, I would say that Shisui is being called the strongest genjutsu user of the Uchiha clan, but I guess you could also interpret it as him being the user of the strongest genjutsu. Him being the strongest Uchiha who used genjutsu is actually the same as him being the strongest genjutsu user of the Uchiha clan.

I would still go for my original interpretation, though. That's the most straightforward, and thus the most likely intended, meaning. Him being the user of the clan's strongest genjutsu would probably be worded differently. Something like 'うちはの最強幻術の使い' or something.

I wouldn't be surprised if Shisui's Kotoamatsukami is considered on par or even stronger than Tsukuyomi or possibly even Izanagi, though.

For one, Kotoamatsukami is unique to Shisui while the other two could be learnt by any Uchiha with the proper level Sharingan. Secondly, Kotoamatsukami is the name of the first gods to descend to Earth. Izanagi is the name of an Amatsukami — one of the gods who came after the Kotoamatsukami. Tsukuyomi was actually born from Izanagi and thus yet another generation later.

Keep your interpretations simple. This is a manga that, although quite deep at times, is still not meant to be analysed for days on end. It's entertainment, not theology."

ShounenSuki:
"My original interpretation was that Shisui is being called the strongest genjutsu user of the Uchiha. Sorry for not being more clear, my head isn't where it should be today ^^;

Also, Kotoamatsukami isn't a single god, but rather a group of five gods.

Izanagi is part of the group of gods that came after them: the Kamiyo Nanayo, or the Seven Generations of the Age of Gods. Izanagi and his wife/sister Izanami where the last of these seven generations.

Izanagi later gave birth to Amaterasu, Tsukuyomi, and Susanoo.

All these gods, together with all the other gods that are descended from, or reside in, the Heavens, are called the Amatsukami orHeavenly Gods. This as opposed to the gods that came from the Earth — like mountain spirits and such — who are called Kunitsukamior Earthly Gods."

Yagami1211:

"From my point of view he's uchiha's strongest "genjutsu user".
and not the user of the strongest genjutsu.

Because : 


最強の幻術使い : The strongest genjutsu user ( the one in the manga )
最強の幻術を使う : He who uses the strongest genjutsu.

The difference is that small."

Shisui:

"Hey, Turrin. So I guess the argument is whether the sentence should read as:

[うちは最強の] [幻術使い][Uchiha's strongest] [genjutsu user]
(The strongest user of genjutsu in the Uchiha)

or:[うちは最強の幻術] [使い][Uchiha's strongest genjutsu] [user]
(The user of the Uchiha's strongest genjutsu)

It's kinda splitting hairs, but the first one is really the only sensible interpretation. The term for "genjutsu user" (幻術使い - genjutsu-tsukai) is a compound word, and an adjective before it should be seen as modifying the entire compound rather than just the genjutsu part.

Here's a similar phrase: 
世界最高の蛇使い
sekai saikou no hebi-tsukaiThe world's greatest snake charmer

What's a more natural interpretation of this line? That this is the greatest charmer of snakes in the world, or that he is the charmer of the world's greatest snake?"

Gottheim:

"Tricky question, that. New powers and techniques are constantly introduced as "teh strongest jutsu evah!!™", so it can get confusing really fast if you take it all at face value. Same goes for characters.
It is perfectly acceptable to translate "saikyou as "a most powerful", as opposed to "the most powerful" in some cases. It is, as many things in Japanese, context-dependent.

Now about Shisui, I believe Bee meant "the", and not "a". Because while Itachi was positively scary in his own right:

1. Unless I'm mistaken (it's not like I know the manga by heart), he was never praised in-story as the strongest genjutsu user of the Uchiha clan. He clearly is the strongest user alive barring perhaps Madara, but this isn't the same as "the strongest ever".

2. The way it is described, Shisui's genjutsu is so subtle you don't even realize you're under it. Itachi's illusions are nigh-impossible to dispel, sure. But you can tell he put you under one right away, and may attempt to fight it (however poorly). Shisui's genjutsu denies you even that. This would also explain why Madara and Danzou were so eager to get their hands on Shisui's eye, and why Danzou thought it gave him a fighting chance against Tobi of all people.

3. Itachi and Sasuke have both demonstrated the ability to put people under genjutsu without them realizing it. But those were simply hallucinations, limited in time and scope. They can trick you into a certain behavior, but that's about it. Your mind is still your own. Shisui's genjutsu is described as permanent (or at least very long-lasting), on top of which it doesn't just show you things that prompt you into a more or less predictable action: it outright controls you, and you don't even know it!

Now, as of the translation itself, here's my take on it:

うちは最強の幻術使い
Uchiha saikyou no genjutsu-tsukai

You mean Uchiha Shisui
瞬身のシスイか？
Uchiha Shisui ka?

The strongest genjutsu user among the Uchiha?

Itachi doesn't bother to contradict him, or even correct him. On the contrary, he builds upon it

シスイの瞳力は対象者が幻術にかけられたと自覚する事なく操る事ができる最強幻術を生む
Shisui no douryoku ha taishousha ga genjutsu ni kakerareta to jikaku suru koto naku ayatsuru koto ga dekiru saikyou genjutsu wo umu.

Shisui's ocular powers deliver a most potent genjutsu that allows him to manipulate his target without them realizing they've been put under an illusion.

t wouldn't make much sense for Bee to say Shisui was the strongest right in front of Itachi if the latter had built a greater reputation in the meantime.
If Itachi had surpassed Shisui in this domain, you could reasonably expect him to have come up with his own version of Koto Amatsukami. But he didn't. He resorted to Shisui's eyes for that instead.

Now you could argue that Koto Amatsukami is the only thing that puts Shisui above Itachi, and that Shisui merely got lucky with this MS powers. But this leades to the kind of convoluted speculation I tend not to feel very comfortable with.

瞬身のシスイか？
 Shunshin no Shisui ka?

Flash-Stepping Shisui?"

Takl:

"the advent of a new susanoO that is different from itachis or sasukes!

scoop 
the astonishing force...!!
Look out for the new susanoO!!
this susanoO Shisui activates is drawn (designed) by masashi Kishimoto sensei specially for NSR.Along with top secret anecdotes about Shisui, enjoy it to the full!

☆3 types of NSR battles to be unveiled!
☆Shisui and Itachi's esoteric technique of fusion, too...!?"

#2349 p.6 28 may 2013
uchiha shisui's susanoO (like [URL="http://en.wikipedia.org/wiki/Steel_Jeeg"]Steel Jeeg[/URL])
↑sｈisui
↓✬cross-sectional surface

#2349  p.7     28 may 2013
Shisui uchiha's SusanoO
the parts in the chest open and from something like a heart SusanoO thousand pieces keep coming out like lasers/razors no stop.
susanoO thousand pieces
blade
 its rotation  might be nice

AnimeNewsNetwork (about the above scans):

"This year's 21st issue of Shueisha's Weekly Shonen Jump magazine is announcing on Monday thatNaruto creator Masashi Kishimoto designed Shisui Uchiha's Susanoo power for the Naruto Shippūden: Ultimate Ninja Storm Revolution game. The Susanoo will be put into action in a previously secret chapter in Shisui's life that the game will depict."

Will add more Official Viz Translations relating to Shisui later.