If you ask me, Id have to say another player.
I mean, the Corperal Beast is the strongest Monster, but I've seen people hit 99s in Castle Wars before. And in addition to that they can have food, and protection prayers, but most PvP areas are not multi-combat. So Imagine taking on the strongest player in the game, with the best armor and the best weapon, who can take you out in one hit. Or the final battle could be you fighting him/her in multi-combat, but you have no one to help you and your opponent would have a level 230 steel titan also staring you in the face.
But I guess it's hard to say who the REAL best fighter in the game would be, but I would say another player.
Players can only hit 90something (not 99) with a AGS special if they have a castle wars brace and is attacking the flagholder. Jad on the other hand, can hit that much at every hit and its also got its own healers not to mention like 1000000000000000000henchmen you have to kill before you can fight him. So JAD
Ice Blitz + AGS
Ice blitz + Dark Bow
All can decimate someone who's maxed in a single moment, with no time to heal.
Anyway, Corp beast is the obvious answer, really. No one has soloed him legit without help from others. Jad is a different type of difficulty, it actually hardly matters on stats, but on the actual player controlling the character. So Jad and Corp both pose a completely different type of challenge, each worthy of being put as the "top dog".
KQ would still own everything 1v1 in Monster V Monster