3M, Norton, Mirka and Sunmight are all good in my opinion.
Also a huge fan of the Eagle stuff, but it doesn't come in big enough sheets for blocking, just for their hand pads and 4 inch block.
In a pinch I'll use Dura Gold just because it is readily available on Amazon in any grit needed.
If you haven't seen the other thread, I launched a line of sanding blocks that have a polycarbonate base and customer reviews are really good.
Not trying to pitch them to you, just an FYI.
www.blackdiamondblocks.com