Best active speaker

ATC specs are conservative (freestanding in open space). You certainly do not get "quantity" from ATC bass - their design is only about quality and approaching the performance of electronics in terms of distortion. The 150's are probably around 0.3% THD from 30 Hz upwards at live concert levels in a moderate room whereas most speakers have about at least 10% and often approaching 50% THD in the LF range - even when played at modest SPL's.

Most people are very surprised at the "lack of bass" from such as large speaker. As you don't get all the LF harmonic distortion that is normal for most other designs and increases the perceptive loudness of the bass - it takes a while to get used to the clean sound of 150's.

ATC speakers starts rolling off in the bass early and more gently than a lot of speakers out there. This will help with room integration as the room will definitely boost the lowest bass by quite a measure thus achieving flatter response in room. ATC speakers also put more emphasis on sound pressure and distortion levels, so as the result they would compromise in terms of bass extension. But really, who nowadays have a 1000 sqft listening room anyway?
What really bothers me is ATC's treble extension. The seas unit they have been using just doesn't go high enough. Sounds more forced than effortless.

Another factor you should look into is that ATC don't put a lot of emphasis on cabinet resonance control. Perhaps aside from the Anniversary 150s, all of their speaker cabinet sings to the drivers quite a bit which is quite baffling since they are so heavily constructed. ATC is definitely not one of those speakers that uses the latest computer modeled cabinet design topology.

Another factor you should look into is that ATC don't put a lot of emphasis on cabinet resonance control.

The freestanding set of ATC's that Pink Floyd use have additional bracing inside. Many studios soffit mount them.

What really bothers me is ATC's treble extension

Indeed ATC focus on the range up to 12 KHz with soft dome tweeter (preferring damped type drivers over rigid types) - this design definitely rolls off earlier than some metal domes like the Be dome or designs with super-tweeter.

These are considered to be excellent:

http://www.neumann-kh-line.com/neumann-kh/home_en.nsf/root/prof-monitoring_studio-monitors_nearfield-monitors_O300#

As for the best, I would say the large ATC's.