Smoothest and easiest focusing manual focus only lens? Hmmmm...probably Nikon's 105mm f/2.5 Ai-S. I've owned two, one I bought in 1982 I guess it was, right when it came out in Ai-S form...still have it...another one I bought in 2004 or so, and later sold to a TPF forum member...GREAT mechanical build on this lens. Built-in sliding lens shade, 52mm filter thread, and well-optimized manual focusing "throw" and a great helicoid. SUPER-easy to focus by hand and eye, good optics, small size, good weight, very nice images. The 105/2.5 (in earlier forms) was one of **the main** lenses Nikon earned its reputation on. I really, really,really liked the 105/2.5 as a manual focus lens for 35mm film work.
Other good ones: 135/2.8 Ai-S, 85/2 Ai, 300/4.5 ED~IF, 400/3.5 ED~IF, 200 f/4 Ai or Ai-S.
I tend to like lenses which are smallish, so the 400mm f/3.5 is kind of an outlier, but for a big-glass lens it its easier to handle than a 300/2.8, due to the slender barrel and the way it was designed to balance on a monopod.