shadow from the games would wreck knuckles. while knuckles beats him in the strength department but shadow is no pushover when it comes to raw power saying he's just an evil sonic is really underestimating him. he's smarter stronger and more ruthless than sonic and a better combatant. I would say he's just as fast but according to "sonic 2006" (the worst sonic game ever) he's not but that doesn't change the fact that he is way faster then knuckles and can use chaos control without a chaos emerald. He can end it with a hyper sonic roundhouse kick to the face then chaos spear.
the archie comics is no better while knuckles is stronger in the comics so is shadow. He runs at light speed and has a direct link to the chaos force. Having a direct link to the chaos force means he doesn't need the master emerald to use chaos energy. Knuckles doesn't stand a chance.
And finally in the sonic x cartoon i'm willing to say shadow is physically stronger than knuckles. since he pushed back a dragon's head that was going to destroy the ship they were on (the entire dragon was about the size of a small planet) so knuckles would be out classed in all fields.
All in all the ultimate life form Stomps
QFT along with the fact that shadow tied up in a fight with penultimate-form emerl.(at this point had the skills of all the combatants including shadow himself, emerl can perfect the abilties he copies and change it into a completely different style, therefore making him unpredictable and heavily skilled.) Knuckles lost to emerl when he was in his development stages, do to his harsh fighting style of not thinking(sonic stated knuckles has allot of strength but is easy to read in a fight.)