Well, as far as I know there are no current slow films that are not relatively high in contrast. Mostly that is because the technology has moved on. TMax 100 is virtually the finest grained film around, with very high resolution and has a very long scale. If you don't like speed, you can easily rate it at 50 or lower with softer development. Acros would be a close second in the graininess category. I find them both finer grained than Pan F. I'd bet they are as fine grained or finer grained than films like CHS 25.
There are those who will recommend the various extremely slow micro/document film options out there with ultra low contrast developers. However even with specialized developers I have seen no evidence these films have anywhere near the exposure scale/lattitude of a general purpose film.