PC/MS DOS of course. It was the last OS I could hold in my head. I could understand every part of the OS. I could write everything from batch files to TSR's. You could reason about every part of it. Did that make it perfect? No not at all but it made it my favorite for a very long time.
Version 7 Unix[1] (the first Unix I ever used) was also small enough to understand well. Since we ran it on a PDP-11/45[2] that had only 256K of RAM and the root file system lived on a 2.5 megabyte RK05[3] disk, it had to be pretty small and simple. We even had the source code for it. I've held on to an ancient copy of the printed documentation for it all these years. I think it's still my favorite OS.