Commander
EDH is where it's at - and by "it" I mean all my money. Most of my Commander decks are themed, and I try to avoid duplicates across them unless it really can't be helped or there are alternative artworks available. This even extends as far as the manabase, so the various 5-colour ones all have different lands to make them feel unique and increase variance when playing (which is one of my favourite things about singleton anyway). This also has a side effect of keeping the power level in check by restricting staple ramp and tutor spells; for example, my Breya deck has several potential infinite combos but almost no ways to actively search for them. I find this creates a better play environment for all concerned as I don't fall into the trap of assuming I'll always have the perfect answer, and find new interactions instead of relying the same cards every game. Sure, sometimes it means that my decks don't function as smoothly as they could and I end up losing but I honestly don't care. That's what Kaalia is for.