My two cents:
It all depends on the amp you're using, but I found that on any cheap (and thus often, but not always crappy) solidstate amp, a DS-1 might improve the distortion tone, if this thing does not create a usable sound, nothing much else will.
On a breaking up tube amp, I love my Marshall Bluesbreaker II pedal, used as a tubescreamer should be used, pushing an already breaking up amp into overdrive, or even further into tube distortion (by adding some gain, or by just adding volume in the boost mode). On a clean amp the Marshall Bluesbreaker II can add a very nice over drive, or nice distortion, depending on the settings.
For 'beginners' I guess any boss distortion or overdrive pedal might do the trick,since the all represent the basic sounds of distortion (although they might not be your 'flavour', if you have developed a more refined feeling for your guitar tone) and it will give you a better feel on how to fiddle with your effects and make them sound good (or if it isn't your 'flavour': make them sound usable).
Do you like fuzztones (and have a amp that does not have a cheap 8 inch or 10inch speaker) try a big muff (altough this might not be the best way to get distinct chord sounds, if you not like perfecting all your settings).
