Eko had so much potential in discovering more of the divine and mystical parts of the island but they unfortunately killed him off a little too early
My thoughts:
Jack: Probably one of the best candidates for leader. He (mostly) makes good decisions, is pretty balanced, and overall did pretty well in the first two seasons. However, when he leads in the later part of season 3, it is shown that he is easy to mistrust due to a number of factors, which include (but are not limited to): Trusting someone too easily, being disconnected with others.
Locke: Locke and Jack are essentially two sides of the same coin when it comes to leadership. Locke hasn't actually done much leadership but he would be almost as good, if not better than Jack. However, some of his faith-based and "the sacrifice the island demanded" approaches might make him kind of unpopular.
Sawyer/Sayid: I have similar feelings about both of them so they are lumped together. Sawyer and Sayid are kind of like Winston Churchill when it comes to leadership, as they make great wartime leaders but terrible leaders otherwise. This is likely because of their more rigorous approach. However, this approach works when there is war going on.
Ben: Ben is overall a very solid leader, as he led the Others for up to 17 years successfully and establishing a nice life for all of them in the Barracks, and he also was able to lead them very well during the war with the Oceanic survivors, as he managed to help them win several encounters. However, his miscalculated attack on the survivors in an attempt to abduct the pregnant women should be considered. During this, around 10 of the Others were killed by the end of it, which could be close to 1/4 or maybe even 1/3 of their numbers. Ben might take too much from others around him and is too dead set on a few specific world view points that it doesn't end up working out.