Everyone reacts differently to different triggers, so it can depend on your individual sensitivities. Triggers can cause different kinds of seizures -- grand mal, absence, atonic (drop seizures), myoclonic, etc. (Drop seizures are different from grand mal, since they don't usually involve loss of consciousness).
Diet soda often has aspartame (Nutrasweet). While aspartame is safe for most people, if you have low seizure threshold it can be problematic, since it is the same as aspartic acid, a neurotransmitter that excites the brain. Foods with MSG can be similarly triggering -- glutamate is the same as glutamic acid, another excitatory neurotransmitter. In addition, if you drink a lot of sodas, the phosphoric acid may mess with calcium and magnesium levels in your body. Both nutrients play a role in protecting the brain.
Stimulants, including caffeine, can be seizure triggers, especially in large quantities. Chocolate is for the most part benign, though it also contains small amounts of caffeine as well as two other stimulants (theobromine and theophylline). Again, it depends on the individual.
Anything that causes blood sugar levels to fluctuate can potentially be a seizure trigger. This includes sweets, caffeine and alcohol. Low blood sugar can be a seizure trigger.
Dietary approaches to seizure control work for some, again depending on seizures and individual sensitivities. Diet modification can range from simpley eliminating stimulants, and generally eating a balanced diet, to going gluten-free. The Modified Atkins Diet (low-carb) has show success in controlling seizures, particularly absence seizures. The ketogenic diet, an extreme high-fat/low carb diet as been proven to help control seizure as well. Because it can have long-term health consequences, it's usually only prescribed for children.
You'll find more info about dietary triggers and treatments here:
http://www.coping-with-epilepsy.com/forums/f39/