The Mark III I used to own was so tight in the lows that it didn't need a OD/Boost at all, as long as the bass control was below 3. It was one of the few older amps that I can plug straight into and get a nice tight high gain tone. They really are great lead player amps. I liked using an overdrive in front of the R2 channel though, for a more marshally kinda vibe. There is some great sounds in those amps.
As far as my favorite boost?? Boss SD-1 is great and cheap. Maxon OD808 is another favorite, but what really surprised me was the old late 70's MXR 6 band I have. It works great to juice the front of a medium gain type amp, I just push the sliders up, accentuating the mids, and keep the low and high bands down closer to flat... basically a frown shape, but the cool thing is the ability to choose what frequencies you can boost with a EQ. Parametric EQ's can be cool too, but are a bit complicated to dial in for boosting... really has great tone shaping ability. Love the Furman PQ-3 and PQ-4, and the Rane parametric EQ's.