They're all good. There might be very, very minor differences between years, but these are differences that make no difference, if you know what I mean. As far as I know, the only slims that are worth significantly more are the gold plated versions, which in any case were called Aristocrats.
I agree with the above. Don't concern yourself about the year, but do note its condition. It's OK if it needs a good cleaning, as long as it's mechanically sound, and the plating is mostly there. Enjoy!
Mechanics/Plating in that order. I have a '59 Fatboy that looked pretty awful when I bought, but it cleaned up and works really well. It still looks pretty bad due to plating loss, but it doesn't affect the way it shaves.
I just acquired a '67 Slim Adjustable, and after using it two days, I already like it better than my Fatboy. It seems a little more maneuverable - maybe because the handle is skinnier, I don't know. I still need to acquire a Black Beauty, and see how it compares.
Unless you want a particular year/quarter because it's a birth year razor, say, there is no appreciable difference between them. As other have said, look for something that's mechanically sound and in decent condition and you'll be fine.