I had used WP Framework quite some time ago, and I liked it. I haven't used Sandbox.
I use Thematic as a base these days, and make child themes under that. I love the way it works. You get a huge number of useful widget areas, and an excellent blank slate upon which to hang your design. It's extremely powerful. You have the Superfish menu built-in for dropdowns if you need them. As far as seriously getting under the hood, I haven't needed to, so I can't comment on how easy it is to hack.
I had also tried out Hybrid, whose forum policy seemed somewhat restrictive. It's a good theme, though. Carrington is very powerful with a staggering number of features, but it's almost too big for my taste. I like to shop for frameworks now and then. :-)
For something already built-out if you don't want to design from scratch, I just installed Suffusion on by blog, and that's really great, very flexible and nice.
I hope that helps.