Based on immortality, Zeref with and without FH and Aizen with Hogyoku are the ones, who are mostly very hard to kill.
The first one survives a complete destruction of his body and resist an Existence Erasure. The second one regenerate from a unknown degree when body and soul are complete destroyed. The only one, who can probably kill both is Kaguya. Why Kaguya? Because her TSB can affect space AND time. The problem with this ability is, it lives more with the statements, then with the feats. If TSB affect time, then even FH Zeref and Hogyoku Aizen would be killed. But that doesn´t mean Kaguya wins this here, since Bleach (Especially Yhwach) has too much abilities, where Naruto characters has never shown resistance.
Oh, and Kaguya has a good regeneration too, but based on showing not in the same league like Zeref and Aizen.
Immortality is imo the most relevant point, because it is a pain in the ass. Even, when someone for example like Zeref is slow compared to the ones from Naruto and Bleach, most of them can´t kill him. Sealing could work, but probably only against the one without FH. Aizen at this point is even more dangerous, because the Hogyoku makes him stronger, every time he came back. It is an accepted NLF ablity, unless someone has a conceptual-ability that can destroy Aizen with Hogyoku, rewrite it, etc. But neither Naruto, nor Fairy Tail has this. At this point, Aizen his immortality in combination with the regeneration is even more effective then Zeref his immortality+time rewind combo.
If they don´t find a way to win, either Aizen w. Hogyoku or Zeref with FH wins this. If they kill, or seal them somehow i would say Yhwach or Kaguya. Probably more Yhwach, because none of them can bypass his Fate manip.