Galaga - I choke as soon as I lose my first ship. Game is usually over within 2 stages afterwards (doesn't seem to matter if I lose my first ship at level 5 or at level 23, same thing).
Robotron - The Tank levels. If I can't down 5 or 6 tank generators in the first second then I am gonna lose 3 men for sure.
Pump it Up - "Come Back" on hard mode. If I play it as my first song I tend to get an A on it. But if it is my second, third or 4th song then I always end up choking on the part where you have to spin around (and thus getting booted off the machine). Problem does not apply to 2 player mode, I can do that one without a miss in 2 player mode.
Pac-Man (speed up)- 5th key
Pac-Man Plus - 8th screen.
Donkey Kong - The entire game.
Star Wars - 4th stage. Never beaten it.
Kangaroo - The screen where all the monkeys stack on top of each other.
Defender - The first level.
Stargate - The 3rd level (I have played stargate more than Defender, thus I am slightly better at it).
Jungle King - The part where you have to jump over the two spear guys. 90 percent of my lives in this game are lost there.
Dragon's Lair - Both of the electrified floor rooms (the one with the Knight and the round one with the thing in the middle).
Super Zaxxon - Inside the tunnel.