id say in terms of pure damage output in a teamfight top 5 are
1. viegar-low cooldowns on spells that can do around 700 damage, plus that ult..
2. leblanc-single target stuff, but that single target is dead
3.karthus-no one really strong spells, but all the AoE, the 1000 damage ult hitting 5 people, and 7 seconds of assured presence in the fight help
4. orianna-constant, low cd, medium damage poking, with cc to keep you in range, with her constant, scaling auto-attacks
5. katarina-2000 damage to 3 champs in 2 seconds? not the highest in the league but its about the fastest dump of damage youre going to see