I posted a thread recently along the lines of "how the hell do I have so many brushes?" finding myself with in excess of 50 brushes! While I do rotate them through and go in phases, I think my favourite is still the Mohawk 5-341 which is the original 1950s model that the PAA Starcraft copied:
I reknotted a few years back when choice of synthetic knot was not that varied, so this is a 22mm PurTech knot which was a kinda copy of the Muhle Silvertip fibre.
Most used is probably this DS Cosmetics professional handle which I fitted a large 26mm Synbad synthetic fan knot:
... over on the left, showing just how big a brush that is. Odd, because I'm not actually a fan of big brushes. Go figure.
Special mention is two-fold.
First, in an age of such choice with synthetic I think it only right to mention the Muhle Silvertip Fibre, which is a superb fibre and enormously underrated:
... and second (but by no means last) the workhorse boar knot from Omega, which I think has the most ideal girth to height ratio in the 10108 handle which is made to a price but by no means cheap in feeling.