I don 't think it's an easy win by any means but Doom just has way better will power feats.

As to anyone else that might be a real battle? In an early story, Reed RIIIICHAAAAAAAAARDS fought Doom in a battle of wills where the winner would control the mind of the loser. Doom won, disintegrating Reed and walking out triumphantly, so happy at finally proving his was the superior mind that he spared the rest of the FF. The rest of the FF were confused since Reed was standing right in the room in plain sight as Doom bragged and left. Reed had actually won and controlled Doom's mind so he thought he won and left without further battle.

Thor might be up there too. The Ghost Rider had used his soul fire on many other heroes and had them paralyzed in fear. Cap overcame it with extreme effort but considered it the most overwhelming fear he ever felt. Even the bravest heroes acted like they were normal humans in a Cthulu story when hit by the Soul Fire. Thor brushed it off like it was nothing, like the Ghost Rider must be joking to even think it would have the slightest effect on a being who walks in the realm of Hela and has beaten the Devil/ Mephisto in his own realm. Not to say Doom couldn't shrug it off just as easily but it might be a good match of will power: Thor vs. Doom.