Fantastic Four I think FF win. With prep time, there is no competition, because Reed has weapons that can crush Omni-Man. No prep time, I think they still win. The Thing has fought Thanos and Hulk (separately) and held his own, who could both beat Omni-Man to a pulp.
AND if the rest of the three can hold him still for a while and distract him, Susan can destroy his organs with a inter force field or give him an aneurysm.
If they can distract him for a while, Reeds just needs a little while to get his weapons, which can surely KO Omni-Man.
Johnny isn't too powerful but he could probably combat Omni-Man for a while, and he has protection because he turns into plasma.
https://comicvine.gamespot.com/a/uploads/original/11127/111270097/6696337-1739085767-RiPjmhDEYfiQUQrZy-xZQ_Lw1UM6LWkHmZrDBjXK_2x5oTehNt9f85TxR_F1Yi0bPlQo9mkhK_vifRsf_O8DePs6o5nzKOpicVWyNHw769B_kLnAibz76aCRVii2wgynFZ7Olg%3Ds1600
AND if the rest of the three can hold him still for a while and distract him, Susan can destroy his organs with a inter force field or give him an aneurysm.
If they can distract him for a while, Reeds just needs a little while to get his weapons, which can surely KO Omni-Man.
Johnny isn't too powerful but he could probably combat Omni-Man for a while, and he has protection because he turns into plasma.