We love Singapore Math and use it every day. I like the problem- solving based approach to it. I don't think the grade level on it is accurate though- my son's singapore book says "fourth grade," and alot of it was covered in his second grade.
We also use Math Minute, which is surprisingly good.
I did break down and have my son do some Kumon. It's boring but he seems to like it, and at some point, you just have to get your math facts down quickly. It's a "drill and kilL" approach.