I usually say Deathfire Touch but that is mostly because I want the Magic Penetration that comes before it on the tree as well. Also DFT does more damage for someone like Veigar than Thunderlords due to the sheer amount of AP he can get.
DFT
8 + 25% AP
Thunderlord's
10/lvl + 10% AP
This means that the amount of AP you need per lvl for DFT to surpass Thunderlord's is:
13.4 (at lvl 1) + 66.7/additional lvl up to a max of 1145.47 AP at lvl 18
Welp, that is a lot more than I expected. I don't really have the numbers at the moment as to when in the game DFT becomes more effective (since I don't know average stacks/AP at each level). I might have to do a game myself for this.