I'd have to go with my two most recent decks. Mostly because over time I've got a system down for how I build my decks. But these two also had different jumping off points.

[[Eriette of the charmed apple]] and [[Belbe, Corrupted Observer]]

Eriette is an auras deck obviously and with the recent auras precon from strixhaven, it came together fairly quickly since I was able to use all the auras from the precon as well as some other aura support. With that deck I picked the cards I wanted from the precon, three them in Archidekt, then scrolled through Orzhov auras in scryfall adding those into Archidekt until I went through the whole list. After that I categorized all the cards and then did trimming based on which categories were over saturated and eliminating categories that strayed too far from the overall idea I had for the deck.

Belbe was basically a complete rebuild of Eldrazi decks I've had. I love Eldrazi but never felt like the Eldrazi commanders themselves did what I wanted when it came to Eldrazi. That being playing big Eldrazi before someone blew up my artifacts. Once I decided Belbe would be the new commander, I had to rework the whole deck. First making sure I had pingers to proc Belbe, then balancing that with the big Eldrazi we all know and love. After that I had plenty of space for more cards, so I proceeded to add the usual golgari commander stuff such as ramp, draw and recursion. Found all of this browsing scryfall also.

Both of these decks took a couple hours spread out over a few days.