I think that Buffy became a good leader by the end of Season 6, though she still made a few mistakes in Season 7. She learned from those mistakes, however, and by Season 8, yes, she is a good leader, and what's more important, the members of her army accept recognize her as their leader.
How that's gonna work out in Season 9 remains to be seen.
On further thought, I would like to amend this statement.
Buffy doesn't want to lead anyone. A line from the original mvie, even though it isn't really canon, sums up Buffy pretty well. "I just want to graduate, visit Europe, marry Christian Slater, and die." A simple, quite mundane, life. Unfortunately, Buffy will never lead a simple, mundane life. She is the Slayer. Unfortunately, she isn't even the typical Slayer, the Watchers' Council equivalent of cannon fodder. Buffy is unique among Slayers in that, for the first time, she is a Slayer who isn't totally under the Council's thumb, even from the beginning, and almost totally independent from them within a few years.
With that independence (not because of it) came anther factor: friends. If the other Slayers before her had very much in the way of friends (Nikki Wood being the only exception that we are aware of) we never knew it. Kendra gave the impression of having spent most, if not all, of her life in training to be a Slayer, to the point that Buffy having friends was almost a shock to her. I seriously doubt that Faith ever had any real friends before coming to Sunnydale, and whatever her relationship with her Watcher might have been, we were never told. We do know, however, that, by her own admission in Season 7, that she was jealous of those things that Buffy had, so it's a fair assumption that she, typical of all Slayers, was pretty much seen as an asset more than a person (we have no idea how close she and her Watcher were, since she never mentioned him or her) What we do definitely know is that Buffy had friends, friends who actually fought alongside her, who did their best to help her, and whom she felt distinctly responsible for and protective of. They depended on her, she depended on them. They were a family. And that was the beginning of Buffy's failing as a leader.
It's a sad fact, but absolutely true, that a real leader, in a life and death situation, has to disassociate himself from the emotion of friendship when that friendship interferes with the mission at hand. Buffy was never able to do that. She risked herself to rescue Giles from Angelus, Angel from the effects of Faith's poison arrow, Willow from Glory, and Spike from the First Evil when the proper thing for a leader to do would have been to accept the fact that, no matter how much she might feel personally or even how valuable as asset one of her friends might have been, risking herself in their rescue from an enemy more powerful than herself could, literally, mean that the opposing side won the war. To a good leader, such a risk would have to be unacceptable.
Season 7 brought a change to Buffy's interpretation of and commitment to being, a true leader. From the beginning of the season, she was the protector of her friends, the Potentials, and Spike, but still not what would actually be considered a good leader. Protecting and defending Spike from the beginning might have been a simple act of compassion or the results of much deeper emotions, but the risks that she took in facilitating his eventual rescue were (brace yourselves, my fellow Spuffies) both unnecessary and potentially disastrous. At that time, Spike was still under the control of the First Evil, was capable of ignoring the restraints of his chip, and capable of killing humans. Whatever asset he might have been as an ally should have been outweighed by the risk of allowing so dangerous a killer into her own home, where, if the First Evil had activated his trigger, would have been perfectly capable of killing several of Buffy's most valuable allies before Buffy could have ever stopped him. The fact that everything worked out fine in the end is no excuse for the risk taken, because she couldn't have known that at the time. Buffy was lucky in this case. Lucky, but reckless.
(Why Giles and Wood were still wrong in their attempt to kill Spike is a subject for a different time, but, still, they WERE wrong, and their actions, if successful, could very well have ended disastrously for everyone and the loss of their war)
Ironically, I think that it was that final risk, or more accurately the person for whom she took the risk, that finally began Buffy's development as a proper, even an outstanding, leader. Two simple statements, "You're the One, Buffy," and "I don't wanna be the ONE," marked the actual beginning of Buffy's rise as a real leader. Finally, as much as she might still hate the fact, she accepted the fact that sacrifices made in a time of war, though regrettable, and never to be taken lightly, are, in fact, sometimes necessary. That friends may have to be deliberately put in harm's way in order to protect the world, and that the mission quite often, DOES have to come first.
That's when Buffy stopped finally came to the realization that it was, indeed, the mission that had to come first, and that keeping the people who she loved had to take second place to that fact.That's when Buffy became a real leader. And somewhere between the end of Season 7 and the beginning of Season 8, she earned the total trust and devotion of her army. That's when she became a great leader.
(And just how that came about is a story that I would truly love to see)
