I disagree with starting with Meddle. I'm afraid that the inexperienced listener wouldn't "get it" by starting off with Meddle. It's a fantastic album, I have the 24K MFSL original master and love it, but I don't think it's the right starting point for a newbie.
Just go straight to Dark Side. Sit in a quiet room, no distractions, dim the lights, put some killer cans on, crank it up, and listen to the entire thing from start to finish, don't bounce around from track to track.
If you watch the documentary dvd about the making of Dark Side, there's a clip of Gilmour being interviewed where he says that he wishes he could be a person who sits back to listen to DSOTM from start to finish for the first time, never having heard it before. Lucky to be you!
Start with Dark Side, then once you are completely blown away and hooked forever, I agree that next on your list should probably be Wish You Were Here.
Remember one thing about Floyd - these are ALBUMS, not collections of songs. Listen all the way through, from start to finish, so you get the full understanding of the message the band is trying to convey.
Next on my list would probably be The Wall, although I've never been a big fan of the second record (showing my age....). But CD1 (back in 2006....) will blow you away. CD2 has it's shiny moments, but I've just never loved the ending. They put out a live cd of The Wall from start to finish, what a great performance. Definitely a must-have as you build your collection.
Animals is also killer.
The Final Cut is a great listen as well, but many really consider it the first Waters solo album. The band never even worked in the studio together, they all came in separately because they weren't getting along (to put it mildly...)
Once you've got a feel for them, then I'd say work your way backwards from Dark Side as well - Meddle, of course, as mentioned before. But also Atom Heart Mother, Ummagumma, etc, and of course the Syd Barrett stuff.
But if you have to pick one cd, without any shadow of a doubt it's Dark Side. Two cd's, add Wish You Here. Then you'll be hooked and find the cash to buy the rest anyway....