Too many overdrive pedals around. And it would really depend on a lot of factors, like what amp you are using, the type of guitar you have, the kind of tone you want, the kind of music you are playing. I think asking what is THE BEST OVERDRIVE is too limiting. As an example, if I use a TS9 voiced overdrive pedal like an SD-1 with a Fender Twin, you get just enough mids to get you to cut through the mix, if I use it with the Peavey Classic 30, the mids are more pronouced giving a different tone.
The best overdrive pedal really is the one that works for you. Mine is an HBE Powerscreamer which I find is one of the most versatile OD pedals around. For the price, the BM is a good choice too. Have not tried the Klon and the Tim which quite a number of people attest to as the "best" OD pedal.