Sensible is a good start.
I would also call Ralph genuine. He has a sincere concern for all the boys that Jack would do well to emulate or copy. He previous ability as a leader of the crew demonstrates this concern, however unfortunately the savagery of the island means this civilized approach won't be as effective.
Another adjective that might clearly describe him might be imperfect. Although he might have the intellect to put together a great idea, he struggles to articulate or clearly express his ideas to the group. Thus, he can't really be the leader that he could have been.
Ralph can be described as brave, fair, charismatic, and a born leader. He (along with Piggy) represents democracy and the struggle for order throughout the novel.