I'm not sure why we're talking about these three players in particular... Harrington is a good all around defenseman and will likely be an NHL player in time... guys that don't play a flashy offensive game sometimes take a while to establish themselves... Babcock is known to favour good puck handlers on the back-end. Percy's skill set should make him a good candidate to play on a Babcock team... I do worry that he's gotten hurt fairly often since he was drafted. Loov probably has the best physical tools... in time he could be an effective hitter who contributes offensively as well... but he made a lot of mistakes as a rookie on the Marlies last year.
Though he's not a threat to make the Leafs for another year or two, Dermott probably has as much or more potential than any of the three d-men listed in this thread title. I also expect Matt Finn to step forward substantially this year with the Marlies... that has been his pattern in his second try at previous levels.
As I mentioned earlier in another thread... I think that Rinat Valiev has the best NHL potential of all the the defensemen in the Leafs' pipeline. He's already got a man's body... and plays a very solid overall game. Reminds me a lot of Dmitri Yushkevich (fortunately without the chronic knee problems).