Welcome to Opeth! As you can tell, there is no agreement between Opeth fans as to best song and best album. Just check the older threads. Although (also, as you can tell) practically all fans would agree that Opeth albums are meant to be listened to from beginning to end, at least when you first start listening to them. Once you start listening, you'll see why.
Personally, I think that Ghost Reveries is a good place for a beginner to start. It's a great album (Despite the fact that it gets bashed here a lot) and is probably their most accessible. As mentioned earlier, My Arms Your Hearse is their most brutal and intense. Damnation is their "mellow" album and good if you like the softer sound of Opeth (all clean singing and it has that laid back 70's vibe). My own personal favorite is Still Life. It and MAYH are their two concept albums.
Aw screw it, ALL of their albums are good. Although I wouldn't start off with Orchid. It's good, but you can tell that they haven't quite matured yet as a band. Just pick one and enjoy. Warning: They become extremely addictive.