I am looking for an OD pedal just to boost the distortion of my marshall, not to necessarily use on its own. What would be the best pedal for this purpose?
Its best to try out some different boosts with your amp. Some take boosts differently than others, and different pedals will sound best with different amps.

That being said, tubescreamer, maxxon, etc..
Overdrive pedals are like fine wines. Everyone person will have a different preference. Some of the best are the TS9, TS808, ZW44, Maxon OD808, Fulltone OCD, T-Rex...... These are all fine overdrives, Each one has their own distinct flavor. Go down to Guitar center and get the accessories Guy to pull them out for you.
Framus Cobra
Marshall JCM 900
peavey 5150

Mesa 4x12