Well man that's an easy and a hard question to answer. Easy because there's a lot of obvious answers, and hard because it comes down to taste. So here's my answer(s) based on what I've used before:
Boss SD-1 (cheapest way to go and sounds great for a boss pedal)
Boss BD-2 (slight more low end and a little less gain, more transparent than SD 1)
Tubescreamer TS9 or TS808 (most obvious answer, sounds great, but does have a mid-hump, which is actually great for solos, but may lack the low end transparency you want)
Crowther Hotcake (the single most transparent distortion on the market, with this one you really have to like the tone of your guitar, because thats what it is, your tone and some drive)
Keeley Katana (simply a clean boost, but through a low wattage tube amp it will def. give you some crunch)
Voodoo Labs Sparkle Drive (well rounded drive, similar to tubescreamer tone, but less mid-honk, uses a special design that lets you mix in the amount of clean tone you want in your sound, so you can get very clean, or you can get very dirty, just depends on how you dial it in)
Zvex Box of Rock (great pedal, very versatile, not enough low end for my tastes, but the clean boost on it sounded great, very sparkly, added rich overtones to my sound, I just didn't care for the gain side, sounded great, nice drive tone, but not what I was looking for in the low end)
Fulltone Fulldrive (Tubescreamer without the mid-honk, but doesn't have a lot of presence IMO)
Fulltone OCD (great sounding pedal, similar to hotcake with a bit more gain and tone shaping available)
Xotic RC Booster (another great sounding clean boost that lets you shape your bass and treble)
Visual Sound Route 66 (good sound, I didn't like the compressor on it, it's a tubescreamer on one side and compressor on the other, I honestly just prefer a standard tubescreamer, but still sounded great)
**MI Audio Blues Pro (tragic showed me this one and I bought it on a whim, MY FAVORITE overdrive I've ever owned, very simple circuit, super small and compact pedal with a wealth of sounds, it has a lot of low end, which I find most pedals lack, and has a switch up top which puts it into a fuzz mode and actually sounds very similar to my analogman fuzzface clone)
Another way to go about it would be to buy a germanium fuzz pedal and use your volume knob to control the amount of gain you want, recommendations here would be:
Zvex Fuzz Factory (just recently purchased one, it has soooo many sounds in it, I finally found a nice super thick high gain fuzz face sound on it that I use to get a great hendrix tone, and on this setting it actually cleans up great and I can get that light overdrive castles made of sand tone)
Analogman Sunface (Dallas Arbiter Fuzz Face clone, what hendrix used, sounds great, cleans up well, just doesn't have a whole lot of drive and "fuzz", almost a clean boost with a bit of very warm germanium fuzz)
