I believe that George is the most optimistic character with regards to his dreams and the future.
When we first meet him, he is building Lennie’s confidence and supports him once again after having to find a way out of a problem that Lennie had created. George repeatedly talks optimistically about the dream with compassion, showing positive care towards Lennie.
George also has a reason to be optimistic as he is the only one on the ranch who has a real friend. On the ranch he thinks positively and sees the work as a new start to another chance, instead of being disheartened and giving up.
Because of his age and being physically fit and healthy George expects he has a long life ahead of him. He therefore thinks he has the time to achieve his goals.
George is strong minded and willing to put in the positive effort into the new start, as well as guiding Lennie through the difficulties towards the dream.
George settles into the new situation easily and has no problems in making friends.
His optimism is boosted when Candy joins the dream making it come even closer to reality. This brings it into a more realistic time frame making it even more achievable. He seems very in control of his destiny, which emphasizes his determination to succeed despite the difficulties Lennie brings him.
Overall I think George is the most optimistic character in the story as he has goals for the future which are achievable and because he is physically and mentally most able to fullfil them.