Loads to choose from! Out of the ones I've tried I like the Fulltone OCD for subtlety and transparency. It isn't over-the-top; it's a fairly dark-sounding type of distortion so it's not fizzly or brittle. There's plenty of volume boost though if you need it, but it's very sensitive to the volume of your playing so the distortion disappears as you roll off the guitar's volume knob...a side-effect of this is it doesn't add much compression or sustain.
If you want an inexpensive, solidly-built OD the best Boss pedal I've tried is the OD-3. That does improve sustain, but again it's not overbearing so doesn't smother the sound. It's not an 'extreme' effect either.
There are others that are popular with people on these boards, like the Catalinbread Dirty Little Secret or the Digitech Bad Monkey...there's the classic Tubescreamer of course, but the midrange 'hump' may or may not be what you're after.
Guitar rig: CIJ Fender Jazzmaster/CIJ Fender Mustang -> TU-3 -> OD-3 -> RAT 2 -> custom IC Muff -> PS-2 -> DD-3 -> RE-20 -> EHX Cathedral -> LS-2 -> Fender '65 DRRI & Mesa/Boogie F-50
Bass rig: Fender MIM Jazz Bass -> TU-2 -> Sansamp BDDI -> RV-5
"...So now you know the true secret to great tone: all the espresso you can handle."