TS-9, TS-7 or Maxon 808.
Which ever you can nab for the cheapest, buy.
The way Tube Screamers shape the sound is absolutely perfect for any kind of rock and metal and they work so well with so many amps too.
I also have the ZW OD pedal and I thought it sounded good, then one day I tried the AMT S1 in the loop of my 3120 with the rhythm channel and man the difference is night and day. I know that's comparing apples to oranges but the ZW OD now sounds very compressed to me and it bugs the crap out of me.