I think that on a ship as big as the E-D, the question "who comes next after main character X" is meaningless. On one hand, the ship will be so full of high-ranking officers that none of the "supporting main characters" would ever realistically get a stab at command. On the other hand, dramatic and budgetary concerns require that these low-ranking main characters always get the job, through some sort of a contrivance that prevents us from finding out who would "really" be in command after Data gets fried.
There never was any situation where Data would have needed a successor, so we never got data on who he, she or it might be. Worf was a candidate, yes - but Kirk's ship also had high-ranking Security chiefs who nevertheless never ever got the chance to try the center seat - the job went to people of junior rank, presumably because Security chiefs have better things to do than command the ship. Worf might thus be disqualified on the same grounds as LaForge. And the E-D would have dozens of Lieutenants in row ready to take the position if our two LtCmdrs were not "it". Or hundreds, quite possibly, as we actually saw those dozens but we also learned the ship had a crew of a thousand people, a (to our eyes) disproportionate number of whom were commissioned officers.
Really, even though we never saw anybody succeed Data, and thus got no solid evidence, we could well argue that Starfleet has no definite system of who succeeds whom below the level of Second Officer. All the officers on the ship appear extremely cross-trained and probably could handle the job; there might be a simple rule there that the most senior officer in sight gets the job in an acute crisis, not the most senior officer aboard the ship or otherwise within the cubic mile. Our heroes would supposedly be above and beyond petty squabbles and wouldn't have any reason to argue no matter who took command.
Then again, see "Disaster"...
Timo Saloniemi