Recently, forum member Dankstar came over to my place, and we did a shootout of all of our OD pedals, to see what was the best boost for my Marshalls and Bogner.
Running through my 87 Jubilee and 84 JCM 800, we tried out the Boss SD-1, MXR Classic OD, Zakk Wylde OD, Way Huge Green Rhino, Klon Centaur, Barber LTD, T Rex Moller, & Marshall Gov'nor.
Some of the fat and rich sounding OD's sounded too smooth or bloated, such as the Klon and Green Rhino.
Those were better on midgain amps or clean amps. The Gov'nor does a good job of turning a clean Marshall into a highgain Marshall, but it's muddy or compressed on an already gainy amp.
Overall, our favorite pedals for taking a highgain Marshall over the top were the Zakk Wylde, SD-1, T Rex Moller, and Barber LTD. The MXR Classic OD and Zakk Wylde are the same pedal with an internal switch that allows each pedal to become the other one. I think I liked the Classic OD more with clean amps, and the Wylde OD with gainy Marshalls. It's also nice that it's not an expensive pedal.
So, go find an MXR Classic OD in green or cream, or a Zakk Wylde. With any of them, you can tailor it to your liking with a Marshall DSL. You don't need an expensive OD to take a gainy Marshall over the top. You only need to be super picky about an OD intended to make a clean amp sound highgain.