Reasons to watch...well, I've seen but a few (can tell you about Only Angels Have Wings and Hunchback of Notre Dame once I've watched them). I will say, however, that TCM is just about the best station I know of. Basically all that I watch.
Stagecoach and Mr. Smith Goes to Washington are excellent, and I highly recommend them both.
The Wizard of Oz and Gone With the Wind are both pleasing, and rightfully famous.
I also enjoyed Gunga Din (a film all Indy fans should see).
As for the year itself, well admittedly, it does have more films in the AFI Top 100 list than any other year (5 - Gone With the Wind, The Wizard of Oz, Mr. Smith Goes to Washington, Stagecoach, and Wuthering Heights).
I really don't think we can say that one year was head and shoulders above the rest, but 1939 certainly may be the top candidate.