Best Overdrive to build -> the question for which a thousand forum threads burned
Best overdrive is entirely subjective. There are better overdrives, worse overdrives, but best... it's a matter of taste.
What kind of music do you play?
Or, what kind of tones are you after?
Maybe with this we can help you better. My fav overdrives are currently the TS9 and the SD-1 (and derivatives), and I play mostly traditional heavy metal/hard rock. The Maxon ones have a really solid reputation, but I haven't built them yet.
It's always good to remember that 90% of the tone is in your fingers (and ears...).