Its depends on what your looking for when you say rare and how much you want to spend on them.
If you want hard to get Chester programmes go for any league games before 1950 or friendly's or Welsh cup games in the sixties and early seventies are quite hard to get hold of.
Now if your talking as rare as rocking horse poo programmes then any pre league thirties programmes or
big games like Fulham or Chelsea at home in the F A cup in 1933 and 1952 if you have big money to spend.
Anyway have a look here this might give you a idea.
http://cgi.ebay.co.uk/CHESTER-v-Wr**h*m-19...1QQcmdZViewItem