Medium Tanks
Medium Tanks are the "Wolf Packs" of World of Tanks. Equipped with enough armor to be dangerous to themselves and a good enough gun to make Heavy Tanks take notice.
Flanking, exploiting gaps, and supporting Heavy Tanks are the forte of the Medium Tank. I prefer Medium Tanks because they fit my personal style of play.
Many people ask what gun they should use to play certain tanks. For guns, some prefer "DERP" guns. They are generally high damage (one-shot kills) but long reload times that must be covered by teammates. Other guns are less damage but come with better penetration (in general) and come with a much shorter reload time. Unlike Light Tanks, Medium Tanks generally do not have the speed to individually circle Heavy Tanks for a kill, nor do they have the armor to stand up to the punishment most heavy tanks can dish out.
As a "Wolf Pack", Medium Tanks are very effective at turnign flanks, but as individuals, they are much better suited to support and exploitation roles on hte World of Tanks battlefield.