See--and first off, I'm a huge Hikki fan. She's one of my all-time favorite artists--I really didn't like that performance. Okay, let me rephrase that: I LOVED the actual performance as a WHOLE. As in, I LOVED LOVED LOVED the set and background, and I ADORED the background music. In fact, the ONLY thing I didn't like was her singing. I would even go as far as to say the singing ruined the performance for me. If I could get just the instrumental I would be one happy camper.
It's just that, well, I hate the song to begin with. In fact, I really REALLY dislike all of Exodus. Absolutely terrible in my opinion, and that's saying something coming from a long-time Hikki fan. The lyrics are extremely bad, in my opinion; completely lacking of any consistency and creativity. They're basically a bunch of extremely short and unrelated sentences put back-to-back to form a song. Furthermore, I just wasn't a fan of her voice in this performance. It wasn't very smooth, to me. That's not saying, however, that she has a bad voice; just that I didn't think it was up to her normal standards in that performance, and it wasn't because of her emotions (at least, not during the entire performance).
And I may sound like I'm bashing, but I'm just trying to give an honest criticism coming from an honest-to-god fan. Personally, this is not a clip that would convince me she's a good singer. There are many other ones, however, that would. This one in particular was just not in my tastes.
As for her having the best voice, like many others have said: that's really a matter of opinion. Heck, the word "perfect" doesn't have a uniform, universal meaning/significance in the first place. You can say it means, "no flaws," but flaws to one person are perfections to others. Personally, I do like Hikki's voice, but I get bored with it after a while, and it's just so...standard. Most Japanese singers with "perfect" voices have nearly the same voice, and I personally like something a bit more unique. If we had to go with a "perfect" voice though, I would choose Koda Kumi in perhaps her song Aishou. That to me is a perfect voice, but many others don't like it.