All of the turtles are, for their comic universe, quite incredibly strong, with additional "supporting strengths" of high endurance and physical durability.
That said, the most consistent references/occurrences of high physical strength and/or "overall strongmanosity" seem to be in favor of Raphael.
(Lookit those muscles! Photo courtesy of http://www.flickr.com/photos/zak…)
The combination of his rage/anger management issues, and propensity to lunge for the center of the fight, gives Raph more opportunities to use direct brute force/strength, instead of the more subtle, tactically-sound techniques of his brothers.
Still, barring Raphael's strength stats in later TMNT video games, or various semi-official or fan-sourced expressions of the various turtles' might in various RPG systems[*] there is very little hard data one way or another.
[* Like this: http://forums.thetechnodrome.com…