We're huge fans of Voodoo Doughnuts in City Walk. There are many varieties and they are a really great value - ranging from like $1.5 - nearly $5 for the big fancy ones. We used to get the Big Pink but now that Voodoo is open, we never do - the VD options are just much better.
Butterbeer is pretty pricey but MAN is it fun. All of the varieties. Definitely have to try them.
Also, I second the ice cream from Florean's - great and interesting flavors.
Volcano Nachos at Margaritaville is a good value and really good with some drinks in a total party atmosphere.
I like the chicken and waffles sandwich and the Krusty burger options at the Simpsons Fast Food Blvd. Also the tacos nearby from the Bumblebee taco truck - those are a great small meal or snack to share.
I also second butterbeer and the Floreans ice cream. I prefer frozen butterbeer, especially on a hot day. Of the Floreans ice cream, I found the chocolate chili to be the most unique to any ice cream I've ever had. The first few bites/licks taste like totally normal chocolate and you'll wonder if they gave you the wrong flavor. Then you start feeling a tickle in the back of your throat. Then as you continue eating it you really start to feel it. It's what I image Fred and George Weasley would sell as a trick ice cream flavor!
Don't skip the candy and snacks in Honeydukes. In their confectionary counter they have cauldron cakes which are kind of like cauldron themed cupcakes that are really good. As a note, the chocolate frog isn't really great, but if it's your first time going and your a HP fan it's nice to get it just for the decorative box and wizard card. The pumpkin juice also comes in a really cool bottle which is nice to display when you get home, but the juice just reminds me of apple cider, nothing too special, and honestly kind of a heavy drink on a hot day. I do like the Chocoballs (I think that's what they are called) which is like a chocolate ball shell with strawberry cream filling.
It looks like Universal Studios is also trying to get into the food festival game like Disney. From what I've seen, depending on the time of year, they have some booths and food trucks set up with unique stuff. I think right now they are doing an Olympic themed event with different country food booths. You can probably find a lot of good stuff in there!
All of the above snacks are really good, but I don't often see it mentioned that there will usually be something unique at the Tribute Store in Universal Studios. Tim Tracker did a YT video on the snacks available right now at the Jurassic Park tribute store, and I think AllEars might have as well. Whatever the theme is while you're visiting, definitely check out the tribute store for what's on.
The Today Show Cafe has pretty pastries, and there are cute themed snacks in Seussland if you want something for the gram. Honeydukes & Weasley's Wizarding Wheezes have themed snacks of all varieties - don't feel like you have to limit yourself to a chocolate frog, haha. We quite liked the pumpkin pasty actually, though the sticky toffee pudding at Leaky Cauldron was better imo. The beef pasties and the scotch egg at the Hopping Pot were also quite good.
Also - churros! I believe one of the QS locations in the Jurassic Park area had them, but I don't recall exactly which one. Or, was it a cart? I'm sorry I don't remember, but hopefully someone else does.
I tried the Apple pie at the Three Broomsticks for the first time and loved it. It has raisins in it though, Luckily I like raisins but some people don’t.
At the Toothsome gift store they have 3 candy bars, one with jelly beans (cherry on top bar): delicious; one with pop rocks (Octo-Popping Bar): different and explodes in your mouth, kids will love this; malt bar (Mechanical Malt Bar): good, melts in your mouth but mild malt flavor.