Classic Rock works really well at weddings because it spans different generations and helps create a feel good atmosphere.  The lyrics are so familiar that most people can't help but to sing along.
Next month I have 2 weddings where my clients requested some classic rock hits.  This playlist features artists such as AC/DC, Lynyrd Skynyrd, and Free. There are some newer additions in this playlist as well such as Linkin Park, but personally I like the classic hits better.