Please don't take offence here, but
I noticed you have just joined a forum mostly focused on objective measurements of systems, not components. There are several forums that are dedicated to speaker system design where you may get more pertinent information.
Is this something you intend to embark on, or is this an intellectual effort just to see how far we have progressed?
By absolute best, but then you say on-axis only as if frequency response was the most important factor, which it demonstrably is not.
Best for what use? Desktop monitors for studio remixing? Relaxed listening in a fully functional living room? Dedicated built HT? The very best tweeter has to be able to work with the mid, which has to work... You get it. For instance a dirt cheap XT-25G on-axis if crossed above 3K wil stomp on many $300 domes. But cross it @ 2K as so many insist on, and might as well use a $1.49 buyout cone. Off axis, it falls flat. The old HDS tweeter has very low distortion, but taming its response is a nightmare. If doing remixing, I may want an boosted mid to listen for defects. Make the worst sound the worst. Think old JBK L100's . So best depends on their use. Some domes are fantastic 15 degrees off, but peaky rising response on-axis. Add a waveguide to a ring radiator and you get not much more than a focused beam. Best "depends"
I am a pretty new at this having only 45 years or so speaker building. Not a lot of them as I was able to build "good enough" about 10 years ago, though itching to try some newer drivers.
It is unfortunate that even price no object, all drivers are so bad they have unique sound independent of implementation. So best may depend on personal preference. Some consider ribbons the "only" tweeter. I don't. Some like wave guides and horns. I do not. I do like the big Martin Logan's, so I am not a box-only guy. To use them, I would have to build a new listening room though. Adding 50 to 100K to a speaker budget is a bit rough.
Drivers that have gotten my attention recently would include the new CSS mid and tweeter, purify mid bass, SB ceramic mid and tweeter, Transducer Technology tweeters, and the Peerless ceramic tweeter. Some like the Accutron seem to have too many compromises even though most excellent sounding. I would like to play with some of the "full range" as mids. When I built my subs, the Peerless XLSS was by far the cleanest, but I suspect many more better ones now. Drivers like the Seas T29D2002 are irrelevant to me. I do not like consumer level DSP processing and I have not exhausted what I can do with passive crossovers, so ease of implementation with passive crossovers is a big factor in "best" for me. If fully independently amped, full DSP of sufficient quality, etc, best may be different.
Many companies have tried to build the best of the best. Wilson Wamm for instance. B&W Nautilus maybe. ML Neolith. I have heard a few and they are all flawed. Better than anything I can build by orders of magnitude, but flawed. And $100,000 for a pair of speakers is a bit hart to swallow even if they are the best. None of these use off-the-shelf drivers.
My next focus is on how clean the 2K to 5K region is. This is where we are most sensitive. It puts a big burden on the distortion of the mid-base as those artifacts are what show up in the upper mids and lower treble. I may have been paying too much attention to the tweeter. Inexpensive drivers like a Mark 4 inch maybe. Even the dirt cheap Dayton RS100 might make a supurb mid if used well. Back to a classic 8 inch 3-way.
So, "it depends"
