For daytime it would be Margaret Howell. I do not own one piece by MH but so much of what I buy secondhand is inspired by her work. Watch this lovely video to learn more about her.
For evening it would be Dries Van Noten. I love his sense of color and luxury. I love how he mixes classics and brilliant new fantasies together. I own one DVN piece — a marigold linen and silk midi wrap skirt that I got secondhand from The Real Real. I have too many favorite seasons to pick just one. This video offers a nice overview.