Given that the first line of VictoryGreymon's profile is that it's a WarGreymon subspecies, I'd be willing to bet the Dramon Breaker has the same properties as the Dramon Killers, so I'm going with VictoryGreymon.
I'd probably give the edge to EmperorGreymon given that he's formed from half of the Legendary Spirits. Then again, In Frontier he kept losing to the same opponents over and over, and aside from defeating Cherubimon hasn't achieved any noteworthy feats by himself. VictoryGreymon has limited appearances, however his defeat of NEO in Next is far more impressive. If I'm going by what we've been shown, VictoryGreymon wins easily.
VictoryGreymon defeated Barbamon one of the SGDLs, on the other side, KaiserGreymon fought against Royal Knight Dynasmon and it took multiple times to reach enough experience to be able to destroy him, so if it's referred to those specific examples winner is clearly VictoryGreymon.
But if one considers this tier system, KaiserGreymon should be the clear winner, since VG is just an alternate mega for Agumon (not a DMO Burst Mode).