After playing with him all week when he was free a few weeks ago, I'm going with Karthus. Lay Waste is great for harassing enemies champs, Wall of Pain is an excellent skill for either chasing enemies, trapping at your turret, or escaping from enemies. Defile is good for the mana gain per kill, especially later on when you can easily take out 3-6 minions with 1 Lay Waste. Defile also has it's fantastic AoE, which coupled with Wall of Pain and a Rylai's is just funny. Then there is his Ult, which at lvl 18 with the build i was using leads up to over 800 damage to every enemy champion: excellent when I'm no where in sight and teammates tell me when to throw it.
So right now, my next investment is Karthus, although im constantly trying out different champions as they become free, such as this week's rather early champion release.