Kuben has possibly one of the best unagi bento boxes in spring valley. I've only got a chance to try them twice so far but they seem to be pretty consistent, and the owner always provides excellent service.
Their bento boxes comes with a variety of items that keeps your palate from getting bored and is organized in a very Snap/Instagram worthy way that is aesthetically pleasing to look at. Almost so that you don't want to eat it... almost.
Their unagi eel bento is very fresh and comes with about 10 pieces over a bed of rice. Each eel piece is surprisingly flavorful and soft, which it makes it very easy to chew and digest. Their salad is also good and comes with dressing separately which is an excellent decision and gives you the choice to decide how much dressing you want or save for later in case you don't want to start eating a soggy salad.
They also did a similar thing with their rice balls, so if you want to buy one and save it for lunch or a mid day snack, you don't have to worry about the seaweed getting soggy and nasty.
They are also updating their opening times starting on September 1st for those of us who aren't early birds. For it's conveniently located spot near where I live, I'm definitely sure that I'm going to come back and try more of their items soon.