In truth, to determine this, you'd have to dissect the most minute discrepancies, since the general point of TNG's very design is for all of them to be the most moral people they could possibly make them. It's almost unprecedented for any of them to be on the wrong side of ethics. So what you end up weighing is minutia. For example...
Dr. Crusher is moral, so wrapped up in the ethical thing, in fact, that it blinds her enough that she's done things I question, like in some cases she puts her drive to heal the sick/wounded above the safety of her crew, (I Borg, & The High Ground) It's a very fine distinction, one not everybody I discuss it with agrees upon
Riker is moral, but is not someone who I think is the most moral of them. He's rather elitist, with people like Sam Lavelle or Reg Barkley, & somewhat short on giving outsiders or lower ranks any kind of graciousness. It was no surprise to me that his actions could've been construed as suspect in the murder of Apgar. Surely he wouldn't do that, but his manner leaves doubt in some people's minds, namely the wife & assistant. Oh, & at the risk of opening that can of worms yet again... he's just reprehensible with Jellico lol
Geordi can be rather graceless with people too, like Scotty, or a few other scientist/engineer types he butts heads with. Worf's morality doesn't really align with our own. Troi's? That's a really fine point with me, because I think that she's eavesdropping on people's emotions is a bit inappropriate, not that she can help that, but she CAN help how she handles it, & sometimes she doesn't do so well with that. The more I watch that Tom Riker episode, the more I feel her & Will just drag that poor guy through the ringer, without ever once really empathizing with him... ironically
Even the ultra moral renaissance man, Captain Picard, who by all rights should be the winner in this debate, is reclusive to an unhealthy degree, & shuts out those closest to him. I've had a similar thread to this some time ago, & the conclusion we came to was that by default Data is easily the most moral. He's the only one who is always examining himself such that he doesn't succumb to even the slightest of those things I mention above. It's in his nature to always be questioning what the right thing to do it is.
Some people even think it's unfair to compare him to the others, as he was programmed to do so, but I don't agree. I mean if the guy can literally give the human Q a fair shake, when no one else would, that really says something imho. He would risk his life for Q. Is it any wonder that someone would eventually write him to make the ultimate sacrifice?