For transistor-based circuits, definitely Electra. Small, easy, sounds great, what's not to like? If you haven't, try making a PNP germanium one! The best recipe I've come up with is with something high gain and low noise - right now I'm working on one with a ~100hfe GT402I, a 1N60 and 1N34 in series to ground for one half of the clipper, and a 1N4148 from ground for the second half. It's surprisingly versatile. With a germanium Fuzz Face in front of it, it gets "Muff Bendery". I like.
For op-amp-based circuits, Distortion +. TL072, 1N60 or 1N34A diodes. Maybe both on a switch! My favorite pedal I have made so far is one of these. I think it outclasses the Rat by miles if I'm using it to drive a tube amp, but it also works great into my SansAmp and straight into the PA. The last show I played was exactly that! It's also fun that the best, most useable setting is cranked.