I'm not scared of complex. A lot of the cards that we consider complex are easy once you've played once or twice with them.
They may create confusing interactions (Ironworks/Great Hall/Trader) but once we get a ruling and a second ruling we're set.
KC and TR with durations still confuse me.
What if I play KC-Wharf, play some other actions, play KC-Wharf again, play other actions, play last KC-Wharf. Now I have to leave all the KCs out there?
And if I play KC-KC-Wharf-Wharf-Wharf I just have to leave one out there? Still a bit clueless.
I don't know what I would consider the most complex card to date, maybe Pirate Ship? But that's because the second clause of the choice comes six lines after the first. Scrying Pool had me confused in the beginning, but that's because I thought: "Wait, it can't be THIS good that I can draw a gazillion cards at once!" But it was.
Possession is confusing because you never quite know who's turn it is: Is it my turn playing your cards? Is it your turn with me playing your cards? With multiple Possessions it can be like Inception. Couple them with Outpost and you're truly lost.
Any card that messes with the setup is confusing at first. Trade Route: You want me to do what now? Every VP pile? Are the Curses a VP pile? They're not. Embargo is similar.
Mint can be confusing because you think: "Do I still get the $ from the Treasures I played or are do I immediateley have $0 left after buying it?"
Potion is weird with cards the specify cost or cost in coins.
The rest is pretty easy.