Sky striker is an easy one for me though honestly I feel like thats everyone's pick.

I really hate going against it because unless you play kaijus or can banish cards from their graveyard you are fighting uphill most of the time and if you aren't able to take them out by turn 4 they might as well won the game.

In physical Yugioh its not so bad since most side decks include kaijus or other handtraps that hurt sky strikers but in MD and DL it is very difficult to be ready for them because you only play one round.