mage:
staff + wizard robes + amulet of magic
(use combat robes from the Fist of Guthix minigame when you are higher level)
ranger:
bow + arrows + leather armor
(use green dragon hide armor when you have 40 range or above)
melee:
there aren't much f2p melee armor that will boost your offensive stats, a few of them are listed below
rune gauntlets
rune berzerker shield
amulet of strength/power