This is a cracking thread and I'm really into suggestions for stuff that I've not heard before. Been trying to get into classical music for the best part of 30 years. I try to get to live performances a couple of times a year and have quite a decent collection of classical music. I'm at the point now where I can make a pretty decent educated guess at the composer if I hear a piece of music on the radio.
I got a book for christmas 20 odd years ago called 'Get into Classical Music' and it was brilliant. It was really useful at identifying music we hear all the time, adverts and films etc and gave examples for other similar stuff to listen to. I've still got the book but it's very worn as I've dipped into it hundreds of times.
As for favourites; probably most things by Beethoven and Mozart. If you haven't seen Saint Saens, organ symphony live, I'd really recommend it. If you can, see it at the Bridgewater Hall and you can really appreciate the reverberations of the fantastic organ.
I'd also recommend Gorecki symphony 3 (sorrowful songs). It's absolutely fantastic.