soldier ME2 shepard will beat halo 3 master chief, here is why, master chief is strong, fast and can only carry 2 sets of weapons a pair of hand helds and a 2handed weapon plus one heavy waepon in both hands which the chief cannot move around quickly with.
soldeir shepard on the other hand hand is not strong put can move as quickly as the chief because of his armor even with for lighter weapons and one heavy weapon. plus shepard soldier has adrenalin boost and concusive shoot.
soldier shepard will out flank and out gun the master chief destroying the shields and aromor and killing the super soldier inside.
the adept shepard will win agaisnt the chief because of his armor again and most of all his biotic powers, throw warp and singularity.
the chief would stand a chance against the sentinal alll the chief will have to do is dodge shepard biotic and tech power that would destroy the chiefs shield and armor, i cant really tell who would win.
the vanguard would certain win with his special move whos name i dont remember. if the chief can recover quickly enough though he can beat shepard, but then vanguard shepard would finish off the chief with shotgun.
infiltrator with his invisibility can take down the chief's defenses and finish him off with and armor penetraing sniper shot.
the engineer the chief stand a chance, it will be a hard fight but the chief will win, the engineers combact drone is not match for the chief.