Lime green, the number 17 (not talking about a card xD ), dolphins, frogs, giraffes.
Ghostricks, Archfiends, Heraldic Beasts, Dark Scorpions, generic Insects, Gagagas, LV, Aromage. Interesting pack-filler cards with potential for amusing plays or trolling potential.
My favorite individual cards in the game are White Magician Pikeru and D.D. Crazy Beast.
I also like supporting underdog themes if possible, like unusual typing combinations (FIRE Fish, LIGHT Aqua, EARTH Winged Beast, etc.)
I dislike strategies that strive for OTK/FTK, floodgating game mechanics that are highly essential, or reacting so hard that they might as well be a floodgate. If a deck's main goal is tailored among these, it's no good in my book, but if it is a deck that just CAN occasionally do it but is not very often, that's another story.
I'd rather use a deck with no killer plays that has a lot of flexibility than one that has very few key plays that it as super powerful but that it has to keep repeating.
Hmmm could update later if I find anything else....