I've got out of the habit of reading as much in the last few years (I more often buy books on Audilble now) but one series I found very re-readable are Lois McMaster Bujold's Vorkosigan novels. There are very few I haven't read at least a couple of times, and I've re-read Memory, Komarr and A Civil Campaign several times.
Aside from that, I'll often re-read (or re-listen) a series when the next novel is due to come out. I did that recently with the Dresden Files series, and before that the Rivers of London series.
