I for one dont see how either would win. Nightmare is not normally vulnerable to physical attack, at least not on Thanos level and Thanos isnt going down to Nightmare either.
Since i am a Thanos fan, and not so familiar with Nightmare apart from few of his appearences in Hulk and Dr. Strange issues, i am going with my inner fanboy's voice and saying Thanos wins.
Thanos wins but Nightmare is indestructible so at best he makes an escape. Thanos would have to be attacked in his sleep in Nightmare's dimension and I can't imagine what fear would plague the Mad Titan to give Nightmare a sufficient advantage. Also Thanos has demonstrated strong mental defenses along with cosmic awareness so he is even more unlikely to succumb to fear in this Fear dimension. Nightmare will have made a dangerous enemy for no reason and likely a future figure of interest for Thanos to investigate and possibly exploit in the future,
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