This is, of course, a matter of personal opinion. My own view is that the most appealing aspect of religion is the fact that it can move people to be extremely kind and charitable towards others.
Having said that, it is important to realize that religion is not the only force that can impel people to treat others with kindness and charity. There are plenty of people who are not religious but are very good to others. However, religion can be particularly good at encouraging people to feel this way.
I would say that the reason for this is that religious people feel that kindness and generosity are mandated by almighty God. This makes them much more likely to follow through and to actually be kind and generous than they would be if they simply thought that good people ought to be kind and generous. If you believe that the being that created the universe wants you to be kind and generous, you are surely likely to do so. When I see religious people being inspired by their religion to go and help the poor or to go personally to help comfort someone who is grieving, I feel that religion is a very good thing. This is the most appealing aspect of religion in my mind.