Greetings Queanh74,
Well I don't know what the "absolute best" is either but let me toss out something for you to consider.
I presume that you plan to sit fairly far back to listen in that large room of yours. In that case, the reverberant field response will strongly dominate the tonal balance. So a speaker that performs well in a much smaller room may sound colored at what would be the normal listening distance in your large room. Several of the speakers mentioned above would have a significantly degraded tonal balance under those conditions. In a large venue (and your room is certainly larger than the average home listening room) smooth power response is more important than smooth on-axis response.
Your requirement for excellent imaging places a premium on a fairly narrow radiation pattern and minimizing diffraction. Good impulse response generally correlates well with good imaging also, though there are speakers with what looks like poor impulse response that image quite well.
The Avantgardes are indeed a good suggestion, and will in my opinion outperform most speakers in a room like yours. Historically getting the powered woofer section to "keep up" with the horns is a challenge, but a large room is conducive to smooth bass which should make it easier to get a good blend between the two. Just don't set the bass level too high, as that will result in a subjectively sluggish bass presentation.
If you might consider a new or demo pair of speakers well within your price range, I have something whose characteristics are in my opinion well suited for an application like yours. E-mail me if you'd like details.
Best of luck in your quest!
Duke
