I'm not a fan of Cillia too much - her style is actually very simple and easy to understand, what makes it sound so great is how she uses multipitch, not the tuning necessarily. And her mixing is top notch. And we'd all die for her PV skills.
BUT I WILL SAY
I ADMIRE SENEI AND ( ゚д゚ ) WITH EVERY FIBER OF MY BEING
BEHOLD THEIR AMAZINGNESS
Senei: http://www.nicovideo.jp/watch/sm18947029
( ゚д゚ ): http://www.youtube.com/watch?v=hcBQ2L1WPe4
and for a visual example: http://www.youtube.com/watch?v=SbeQrKfkRkw
though their best stuff is her shinji things which she rarely shows us the pitchbends of
wish ( ゚д゚ ) had a mylist or at least let us look at her vids on nico orz and damn she needs to release that unnamed utau of hers dear god i want it so bad the potentiallll
I think they're two of the best UTAU tuners in the fandom, period. Everything they do sounds so amazingly realistic ahhhhh~
On a level closer to us measly humans, I also admire Yoyopines/Yoyosama - his UTAU tuning, while not overly complex, always sounds great and GOSH he can make Sinsy sound good. http://soundcloud.com/yoyosama-1/
WITH VOCALOID everyone knows that Ciel is god, I mean DAMN those VSQs are nice, how do they even draw those vibratos that beautifully?! Frutella is awesome as well, MitchieM either uses Vocalister, Vocashifter, has good references, or is a god, and while Kanon746's stuff isn't as hyper-realistic as the other examples, with how difficult Vocaloid is to tune I'm always impressed with how great it sounds.
I'd bring up whimsoft (http://www.youtube.com/watch?v=4KjSJuRDz64 ) but I'm 99% positive they use a program like Vshifter - still impressive if they edited that all by hand unto Vocaloid with it as a reference, and admittedly still kind of impressive if they put it on Iroha and adjusted it to make it sound as freaking gorgeous as it does.
IN CONCLUSION for me I'm most impressed and struck by tuners that achieve realism. I do appreciate good UTAU/Vocaloid sounding tuning though - I like Yoyopines and I don't dislike Cillia or anything. But what will always strike me best is the best of the best. Though I'll never get there, I'll just always be one of those average UTAU type tuners.
*stares at post* sorry i had a lot of passion about this topic