I'll try to give you my experience without writing a book.
It really depends what you're going for. I started with the Sansamp BDDI 3 channel pedal. Loved the tone, ended up building a rack and got a Sansamp RBI since 1) it was pretty much an always-on effect, and 2) the RBI rack unit had a mid control (unlike the pedals).
Then I combined it with the MXR-M80.
Eventually I moved to a Darkglass B7K and that became my always-on pedal for tone sculpting.
Here's my input.
1) the Sansamp is great for that tube overdriven tone. I still revisit it here and there just to see if I wanna switch things up.
2) The MXR-M80 is a great pedal, but the distortion on it is WAY too extreme. I don't really consider it an overdrive because the grind it gives is just way too harsh. You need to dial the blend back to a)keep your bass's natural bass in the tone and b) to dial out some of the harshness of the distortion. Also, the clean channel eq is nice on the M80, but the "COLOR" button on the clean effect will scoop your tone. I upgraded to an MXR M81 so I could just have an EQ at the end of my pedal chain. Basically the clean eq section of the M80 but with more controls.
3) The Darkglass stuff is more of a modern sound. If you're looking for a little more clank and punch in the mix, this is great for it. A lot of us DG guys are using the B7K as an always on pedal with the drive set pretty low.
4) I'm also using a B3K as my distortion on top of the B7K.
5) I tried the Darkglass VMT, and I'd compare it to the Sansamp, but it's just got its own flavor and isn't necessarily the same. Think of it this way, if you want a Darkglass pedal with an Alice in Chains grit, get the VMT. If you want a Pantera or heavier grind, the B7K/B3K is what you want.
Out of all of the pedals I've run through, I'd say the B7K and Sansamp RBI are my two top choices. They do different things, but do them both well. The only pedal I've tried and hated was the Boss OBD-3. I bought that and sold it on Craigslist 2 days later.
There's tons of things on the market that you can test. Best thing to do is pick a few of the top suggestions, and research the living hell out of them on Youtube finding samples.