If "beginner" means a player with enough knowledge (on item building and other stuffs), i would recommend INVOKER and GRAND MAGUS.. i was just joking! Try these:
Lion, Rylai, Sven, ES, VS et al: Stun, slows, and disables come in handy. Good for ganks and escape.
Slark, Davion, Omniknight: Built in heal and regen
Axe, Centaur: High HP
Ursa: More life = More damage, survivability = ownage
Jah'rakal, Mortred, darkterrror, Traxex, Spectre, et al: Good killers though farming is vital.
Avoid invoker, magus, potm, techies, pudge...