I would recommend that you avoid the brisket if you typically go for that. I was very disappointed by it at Nats Park. I miss the brisket at RFK.
If you want a good dessert and not just soft serve ice cream, look for the gelato stands. The gelato is comparable in cost to the ice cream, but it tastes better and has a better variety of flavors.
I haven't sampled too much there by way of food. The cheese steak is pretty good and seems like a good value, but it might be kind of odd if you've only ever had cheese steak with actual cheese slices and not whiz. I'd only ever had cheese steaks with provolone/American cheese slices melted on, so having some cheesy sauce-like stuff on it was unexpected. It's decent in taste, but it gets cold kind of quickly.
If you want a good hot dog (must...resist...urge...to make...Elijah Dukes joke), get the Hebrew National dog. It costs more than the regular, but it's miles better.
But yeah, there are tons of choices, so the best thing to do is take a good, long walk around the concourses and take in the sights of the stadium while getting a good look at all of the food options. No sense in limiting yourself to one thing when you have a whole litany of options.
Another suggestion, if you're too poor to afford anything but crappy nosebleed seats, is to go over to the big staircase structure around left field. There are various points in the thing where you can stand over the rail and watch the game for a good half-inning or an inning. When it's late in the game, security generally doesn't care if you do that. They have some really good views on that thing.
