Sorry for being a smart**s...we're just a goofy bunch in here

. My suggestion (and I've owned
lots of everything) is start with a Big Muff, be it a Little Big Muff, Russian Big Muff (my first fuzz), or any BM derivative. I think they are great bass fuzzes, except for the mid-scoop which is characteristic of Muff's. I've not owned a Rat or Messdrive, but I think you could not go wrong with either. I think between the two, the Messdrive is the better choice, based on clips I've heard and the reviews (check out TB'r Grygrx's excellent site bassfuzz.com).
I believe what joeinsprings said certainly applies as well. You may have to buy/sell/trade many pedals until you reach your target tone. Again, that's something I've done and I'm still not there yet. The main thing is to try them through your rig and live with them for a few days before making a decision. The other important aspect is how a fuzz/distortion sounds in a band setting, as opposed to solo.
Hope that helps,
Lyle