So what is the best speaker you have ever owned?

Hard to choose one but my favs since 1959 are Dynaudio Sapphire (dealer disclaimer), Ohm F, Apogee Duetta Signature and double stacked Quad 57.

Reference 3A De Capo's. After BW602, Triangle Comete, these suckers sing, great dynamics at low vol. levels, rich midtones and solid bass for a stand mount.

My Dynaudio C1's. Especially since I replaced my ss integrated with tubes (Octave V70SE).

This is a good question but the answer is relative to so many factors. Price, room size, musical preference and amplification to name some obvious ones.
For me it's a toss up between Meadowlark Audio Kestrel Hot Rod and the Soliloquy 5.0 monitors. I've owned speakers costing 3 times the price of the Kestrel HRs and sold them all after a few months or so.

ESP Concert Grands and Genesis 350 SE (also really liked Coincident Total Eclipses).