obi wan stomps if we count the EU and novel. if this movies only then itachi stands a chance. but still loses.
obi wan as a padawan deflected and reacted to lasers being shot at him and his force speed actually avoided said laser.
his telekinesis is capable of dropping down heavy rocks on itachi forcing him into susano.
susano should be cut apart by a lightsaber which slices heavy metal armour (the kind of metal they put on star destroyers) like butter.
amaterasu is lifted up and deflected by the force. obi can lift and deflect much heavier things. galen marek who should be weaker than prime obi wan (he was equal to decrepit suited vader) tore a star destroyer out of the sky, and caught a tie fighter mid fight. the force unleashed is stated by lucas as canon to the movies so no one better tell me its invalid.
theres also force crush that ignores conventional durability because it can crush your organs. and force choke which is admittedly a dark side move. but this is a fight. im sure obi wan can get his superiors to forgive him.