Morgana is one of the cheapest, and most powerful. Strong farming, good Shield against Casters, and can set off a perfect combo of her Q > W > Ignite > R. Which isn't hard to pull off. However if you play on Draft, shes banned for the most time for that reason and more. Other than Morg, LeBlanc is pretty good too. Kind of like Kassadin, except her combos are more unexpected. After LeBlanc, Annie. Another great Champion, with a strong combo and great passive. Its up to you mainly. Everyone has different styles that suit them, so why not try them out on their free week?