An A list title for me is a title that doesnt come up for sale too often due to its gameplay/collectability/artwork. It is out of reach for most of us due to its limited numbers/rareity and price tag. It is the "ferrari" of pinballs.
Apart from that, its the pins that you like the most. The titles that you would never sell. For me its Pinball Magic,GnR,Banzai Run,Funhouse. I can list the most popular and overated IMO, but we weill leave it for another thread.