Why would Samneric be the best candidates for being chief on the island? What evidence from the text supports this idea?

The characteristics Sam and Eric, the twins, possess that could make them a good chief (or good co-chiefs) are their cheerfulness, their willingness to work, their values, and their compassion. When they first appear in chapter 1, they throw themselves down, grinning and panting like dogs, their "cheery duplication" a pleasant surprise. They get the crowd of boys to laugh. Samneric don't seem to mind working hard, as long as they can do it together. They carry the slain pig between them on their shoulders in chapter 4; in chapter 8 when all the biguns except Piggy and themselves have deserted Ralph, they sneak away with Piggy and present a huge array of fruit to Ralph to cheer him up. They also work very hard on the fire in chapter 10 before they realize the job is just too big for only three people. 

Samneric display good values for the most part, staying loyal to Ralph until they are kidnapped by Jack. They have the perception to realize that Jack's way isn't right; when Ralph mentions getting taken prisoner "by the Reds," Eric and Sam say that would be better than getting taken by Jack's group. When they do get kidnapped by Jack, they warn Ralph, telling him that Jack and Roger are "terrors."

Finally, Samneric show compassion toward Ralph after Piggy's death when no one else will. They try to warn him of Jack's plans, and they pass him some meat. They end up being beaten for that.

Although Samneric have good qualities, like cheerfulness, willingness to work hard, moral values, and compassion, they nevertheless have some qualities that would interfere with their success as chief. First, they are too dependent on each other and because of that are a liability to the group; Piggy points out to Ralph that the two of them only take one turn at the fire and that isn't fair. Second, they tend to be easily frightened. They retreat from Jack in fear when he first paints himself, they obey him when he takes them away from the fire to hunt the pig in chapter 4, and they race down the mountain in terror when they see the dead parachutist. In the end, they give away Ralph's hiding place to Jack when he physically punishes them. 

Samneric have some good character traits, making them a better choice for chief than many of the other boys, but they also have qualities that would detract from their ability to lead.