Most fun? Any table 500 hot garbage with obscure shenanigans or convoluted infinite loops. But you won't win games because your opponent will always have unbreakable boards T1 or five handtraps T2.
Honestly it kinda depends on your playstyle. I personally like to be extremely aggressive or to find silly ways to stun-lock my opponent. And I personally despise playing meta so my main decks are Skull Servants, Majespecter and Mayakashi. Occasionally some Lightworn or Blackwing when I need a bit more of competitive push. Since Block Dragon and Brilliant Fusion just got off the banlist I might go back at playing Gem-Knight.
But sometimes I mess around with nonsense like Tiger King Wanghu + Al'miraj, Ghostrick Subterror, Empty Jar, etc.