The MDR Sinfonieorchester is the symphony orchestra of the "Mitteldeutscher Rundfunk" (literally "central-German Radio", but it's more accurately the local radio and TV station covering the eastern parts of Germany) and it's based in Leipzig, making it one of the two big orchestras there (alongside the much more famous Gewandhaus Orchestra). They're mostly also performing and recording at the Leipzig Gewandhaus hall.
Although I probably wouldn't quite rank them amongst the top ten of German orchestras, they certainly are not far behind and as with all German radio orchestras, their standard is of course one of highest professionalism and their recent recording projects have all been very ambitious.