Best for what? For lead work, when the guitar sound needs to cut through the mix, nothing beats a good old Ibanez TS 808 type overdrive pedal. There's a reason why that circuit is a classic. I've tried the TS9 version but I could not stomach it. It's got to be 808 specs.

Having said that, I tend to prefer "singer-songwriter" moments of just someone's voice and the guitar, and I find the TS overdrives inappropriate for that application (or for rythm guitar work, generally). So if I'd have to pick only one circuit and be forced to choose to play just one way, the clear winner would be the Way Huge Red Llama circuit. It doesn't boost the mids, it overdrives the full signal (going into fuzz territory), and it's the best solid-state simulation of an overdriven 5F1/5E3 (think Fender Champ / Deluxe) tube amp I've ever heard from a pedal, bar none. So that's "my best".

Unfortunately Way Huge went bust a while ago. The brand name has been bought by Dunlop I think, and they chose to reissue some of the Way Huge pedals, but not the Red Llama, and so the world being what it is you can only get an original WHRL for a handful of hundred dollar bills. BUT, there's still hope, because there are Tonefactor Mule / Mojo Hand Mule pedals still around (also out of production, but still to be found on eBay), the Greedtone boutique OD pedal is apparently based on the Red Llama circuit, there's a also a guy named Steve Liberty that makes a clone called Llama Roja, and for the tecnically inclined there's a dirt-cheap kit available from General Guitar Gadgets.