The factory DAS is the best (naturally). Other than that I suppose the best aftermarket scanner is the Baum ISCAN. I own one and am very UNhappy with it. It does not read all of the systems it is advertised to and gives incorrect fault codes many times. I'm sorry to say it, but its still probably the best out there other than factory. For the do it yourselfer I would recommend a generic OBD 2 scanner but it will not get into the subsystems.
Are there any others out there as unimpressed with the ISCAN or CS2000 as I am? Just thought I'd ask.
Mercedes Benz Master Tech
ASE Master Technician
Owner of "Behind the Star Inc."