Well, just my opinion of course but...
The best tweeter/high end I’ve ever heard is still the MBL radialstrahler
omni design.
I still distinctly remember first encountering them at CES in 2000, wandering the grounds I suddenly heard for the first time what really sounded like live music coming from a room. I entered and the MBL 101Ds were playing some jazz. Aside from the obvious imaging and clarity, what struck me was the realism of the drum cymbals - they were actually full, big, round, dimensional and shimmered in the air like real drum cymbals. In contrast, every other speaker at that show, or that I’d ever heard before, sounded squeezed, like "tweeters."
Subsequently I heard various iterations of the 101s in better conditions and that aspect of their sound reproduction always impressed me. I finally got hold of a pair of MBL 121 monitors several years ago, which I keep around even as I go through other high end full range speakers. Every time I play the MBLs they just embarrass just about everything I’ve heard in terms of natural high frequencies - the most effortless, natural detail I’ve ever heard. I know Jonathan Valin of AS touted the MBL tweeter as the best he'd heard for quite a long time (though I think he's described some new speakers as even better).
(I recently auditioned Raidho speakers, thinking of buying a pair, and their high end was indeed smooth, but overall they didn’t impress me as much as the MBLs).