Deathfire Touch or Thunderlord's Decree?

Depends on what you want. Hybrid champions tend to take thunderlords to go the hybrid pen. If you are a spammy champion the 5% cdr can be helpful. But if you are a caster like Ziggs or Brand I suggest that you go DFT

They're both good, depends on the champion. Thunderlord's got nerfed, though.

As I take thunderlords, for the 5% cdr and better single target burst early as I have all the aoe i could ever need

But there is no which is better overall, it very much depends on champion and playstyle

Thunderlords is great early and for helping destroy one target Deathfire is great late as a little extra damage all around

if its not single target its not worth taking DFT

Jhin and Malz make good use of DFT.

Other than that DFT is pure garbage.

DFT if you have low cooldowns and want a little extra damage in between cooldowns / have single target spells and plan to stack damage

thunderlord's if you want to kill someone as quickly as possible