Word for someone who "sees the best in others"?

I think it depends on the person and his/her perspective on the matter.

For instance, someone who recognizes that humans are flawed but believes in their fundamental goodness nonetheless could be described as forgiving, good-natured, tolerant or benevolent.

Someone who has faith in others to do the right thing (and potentially doesn't see or overlooks their character flaws) could be described as trusting or idealistic.

If we, knowing as we do that people are rather nasty creatures, describe such a person from our cynical vantage point, we might call him/her credulous, naive or unsuspecting.

You could call someone a Pollyanna. Although its meaning is a bit broader, it might be appropriate in context.

I would say the person has a 'generous' view of the other.

Such a person might be said to be sanguine in nature, or possibly be a Pangloss.