If the highest gain needed is for hard rock like Guns N Roses, that actually pares down what all you'd actually want to consider, nicely. Also, for hard rock, I think of distortion with lots of mids, and probably a bit less bass. So any drive pedals with a flat EQ, scooped mids, or heavy bass can all also be excluded. Likewise, if the pedal is an OD proper, and is incapable of more toothy aggressive distortion, it can be excluded. IMO, after you exclude everything above, that puts the Rat at the top of the list. It will obviously do hard rock, but also will do mid gain and low gain stuff, too. In 2nd place, I put a DS-1 modded for midrange, and with the clipping/distortion modified to be less buzzy/fizzy. These changes will make it especially work better at medium and lower gain settings, IMO. Also, adding mids will make the pedal much more flexible with the vast majority of clean amps, which tend to have flatter or scooped EQs. Runner up would be either a Marshall Guv'nor or Shredmaster. The Guv is going to be the one you want for medium or lower gain stuff, and it's obviously EQ'ed for rock-type stuff. The Shredmaster's strong suit is the harder high gain rock, and has a contour control that lets you sweep from scooped to mid-emphasized, also adding a bit of flexibility with whatever clean amp you might end up using it with. ...Most folks would probably end up with some kind of clone or variation of the Guv or Shredmaster. The Crunch Box and Riot are just two flavors of Guv that have the singular tone control, if you don't think you'll need to really fine-tune the EQ. The Joyo High Gain Distortion is a low cost variation of the Shredmaster (with the only caveat being that you can't adjust the bass). The Barber Dirty Bomb takes the Shredmaster to an envelope-pushing place, and probably could be used for metal, too.