Me, me, I know, I know
I thought I knew but then I can't do this with a single answer; any of the Pflueger Medalist reels, the Orvis Battenkill reels early ones made by Pflueger and then by Hardy and BFR. The Orvis CFO reels were hard to beat and I still use them as well as my Hardy reels from the Lightweight series.
In other words there are many of the best trout reels made, I tend to focus on older issue reels because they were the ones with tens of thousands of users.
I guess I left out all those Hardy Perfect users, Hardy made the first in 1891 and are still producing them. They are as smooth as it can get in a closed cage reel I think.
Obviously I can't answer the question