That’s actually a tough question. I like the screen pass you cite and a slant and always enjoy a well executed bomb that the receiver catches in stride. A well executed flea flicker, direct snap or other gadget play is usually exciting, but pretty rare. I know I don’t find QB sneaks exciting.
I would have to say that my favourite offensive play to watch is actually a defensive play, namely when a pass rusher cranks the QB and causes a fumble. The scramble for the ball can be very entertaining as well. I used to also like watching a wide receiver catching a pass on a crossing pattern and immediately getting smoked by a defender, but this is now illegal although it does still occur at times.
And this is not because I am a sadist. I just enjoy the nature of a good clean hard hit in the roughest team sport and the fact that the person hit more often than not gets back up. And without whining. This to me is the essence of football. Unlike the pussies in hockey where the rule is now that no clean hit should go unpunished.