That's a great question actually...Makes a person think about what really is the greatest reward of their situation. As most people would probably agree, it'd be easy to list off all the hardships and limitations they find themselves in, but to narrow down the greatest reward of it all certainly provokes a person in to thinking a bit deeper.
I would have to say that the greatest reward is one that I will never be around to actually witness, but truly hope it happens. Reason being, is because if/when it does, it will mean that I actually did do what I was supposed to for my girls every step of the way. There are many things now I that I find to be rewarding. This answer will be hard to explain actually, without it seeming like I am minimizing them. So before I proceed, I would like to say that my intentions are, in no way, to minimize the rewards that I have been blessed with today, as they are exactly that, rewards. I think the greatest reward though, and as it weird as this may sound, comes after the time comes when I am no longer here. When I am gone and am no longer with them here on Earth... I hope, with all my heart, that they can say sincerely, to anyone who asks, that their Dad; loved them, cared about them, adored them, worked for them, taught them...That their Dad never held anything back and left everything he was on the battlefield of life, every day of his life, in order to provide the best life he could for us. If that is something that they can say when that time comes, my spirit will at that time will feel the greatest reward of being a single father.
Perhaps the answer is a bit deeper than what you may have been looking for, however that is how I feel...I think the short version of that, one that would put the answer in the here and now, is their love. What could be more important to a father than that..? I know it isn't money...It's not social success...It's their love.