Ax of Animorphs and cinnamon buns. Considering his adoration of chocolate, chili, and cigarette stubs, establishing one thing as his trademark favorite food is quite a feat. One of the subtle differences in the AU bookBack to Before was that Ax was never introduced to cinnamon buns, and instead fixated on Oreos (possibly as an Homage to the Martian Manhunter) as they were given out at the mental institution he was in. (Don't ask.)
While cinnamon buns is Ax's absolute favorite, his favorite things to eat all happen to start with the letter C.
In The Dresden Files universe, the lesser fae (wyldfae) are absolutely insane for pizza. In fact, in "Small Favor", they have even formed a Za Lord's Guard for Harry because he's given them so much pizza, and refer to him as their "Lord of Pizza". And in the next book, Turn Coat, he raises a militia of the little guys. With only the promise of a large pizza for every twoscore lesser wyldfae to flock to his "banner". This probably makes him the only mortal to receive two titles to the fae, as he is now also the winter knight.
Also, Harry loves his Coke. Sometimes it seems like he runs exclusively on it. He even has a cap with a silver-on-black Coca-Cola logo. He also really likes Burger King.
In Larry Niven's Known Space novels, potatoes should probably be this to every human over a certain age, as it's more or less stated that tree of life evolved into potatoes strangely averted, however. The Virus is responsible for that effect as well.
In the Discworld novels featuring Susan Sto Helit, Quoth the raven is obsessed with eating eyeballs, but he tends to mistake vaguely eyeballish foods like pickled olives and walnuts for his favored treat.
Rincewind and potatoes. This started as a Running Gag in Interesting Times, but has become a fundamental part of his character since. In Science of Discworld II, when the ElfQueen tries to find his deepest desire (dismissing the "stupid tuber"), she comes up with "sour cream topped with chives". This stems from an extended time Rincewind spent marooned on a desert island. He was safe and had food, but the two things he lacked were female companionship and potatoes. Some wires got badly crossed in his head.
Tiffany's little brother Wentworth really, really likes sticky candy. Tiffany herself is quite fond of cheese, enough so to feel tempted when it shows up in a dreamworld where she will be trapped forever if she eats or drinks anything.
Senor Enrico Basilica (from Maskerade) is assumed by everyone to really love Brindisian pasta, and is given it at every opportunity. Since he lies about being from Brindisi, this is not actually his native food and he's, at that point, sick to death of it.
On the other hand, Bengo Macarona, a Genuan transplant in Unseen Academicals, actually does like pasta, not to the point of bringing it up frequently but certainly requesting it during one of the wizards' extended food-ordering sequences. He also keeps dried reheatable pasta in his room for emergencies.
Susan's mother, Ysabelle, had a chocolate addiction so strong that Death considers it one of two character traits worth mentioning to her daughter. Susan herself seems to be developing one in Thief of Time
Nanny Ogg's Cookbook strongly implies that ham rolls are Gytha Ogg's party food of choice, as she drops numerous hints that having a few of these on hand for any surprise-guest witches who might happen by would not be amiss. Sometimes more than once in the same paragraph.
Even though he doesn't need to eat, Death likes curry, to the extent that the Curry Gardens in Ankh-Morpork now recognizes Death's trusty steed Binky on sight.
Dumbledore and sherbet lemons (or lemon drops depending which edition you're reading) as well as candies in general, except for Every Flavor Beans.
Harry and treacle tart, plus pretty much any dessert or sweet with treacle in it.
Remus Lupin and chocolate, if the fanfics are to be believed.
Karen Traviss's books in the Star Wars EU, any and every character who has ever tried this one dense syrupy cake made by Mandalorians has it as their absolute favorite. They say something along the lines of "It's good, but it's not uj cake" approximately every time they eat any other food. It looks like Traviss uses uj cake as shorthand for how incredibly awesome and superior Mandalorians are supposed to be.
Same universe - the X-Wing Series has Corellians and ryshcate, which is only for special occasions; traditionally it's shared to remind Corellians of their homeworld and to smooth out grievances. It's hard to get the right ingredients, which are expensive, so it only shows up a few times. While Corellians really like it and so do the people they share it with, it isn't uj cake.
In The Thrawn Trilogy, Luke Skywalker mentions that Lando introduced him to an exotic beverage called hot chocolate, which he enjoys. Later-written books and comics will, if they don't make up a new drink, also have him ordering or making hot chocolate, and he liked it enough to stash some of the pods needed to make it in his brother-in-law's freighter. His wife, family, and various friends also come to like it. But it isn't uj cake.
The Kid from The Stand, is obsessed with Coors beer and Rebel Yell whiskey. Interestingly, in the story he drives right down I-70 in Colorado, which would take him practically within sight of the brewery itself, but it's not mentioned.
Alcoholic Mook Krager in The Elenium has a trademark favourite wine, Acian Red.
Whether it's the bones, or served buffalo style, in ramen, or katsu, Malik ibn Ibrahim from "Wandering Djinn" always seems to be eating chicken.
Dexter is a big fan of good food in general (he heaps praises upon Rita's cooking), but has a thing for cuban sandwiches.
Shows up a few times (of course) in Honor Harrington. Vying for "most notable" are Honor's predilection for hot chocolate, and the absolute adoration of celery that Nimitz shares with his entire (carnivorous!) species. Meanwhile, Honor's best friend, Michelle Henke, is later on shown to have a terrible fondness for doughnuts. Other noticeable standbys are Old Tilman Ale (seemingly the only beer Manticoran officers ever share), and Delacourt, the vintage of wine that Honor breaks out to celebrate any really special occasion.
Pumuckl: The kobold protagonist of a German children's series craves chocolate (but who doesn't?)
Maximum Ride developes a near-obsession of chocolate chip cookies after having some home baked by Ella and her mom. To a point where she'll evaluate a cookie after eating it and is still trying to find some as good as the ones she had.
In Ranger's Apprentice, all of the Rangers have an addiction to coffee and manage to get a few of their non-Ranger friends hooked on the drink as well.
Adrian Mole has a fondness for Mars bars and Lucozade, at least as a teenager.
Bert Baxter and pickled beetroot. In fact, Bert will only eat beetroot sandwiches, scrambled eggs, boil-in-the-bag curry, and butterscotch flavour Angel Delight.
Born to a family of bakers, Peeta in The Hunger Games takes a strong professional interest in bread, which the fandom has amped up toCrack Pairing levels, fed by the fact that Katniss' narration refers to him as "the boy with the bread" several times.
Katniss once ate lamb stew and mentioned how much she liked it. She is then given stew several times throughout the books.
Sam in A Tale of Time City loves Forty-Two Century butter pies. Actually they seem to be everyone's favourite food, but Sam will cheerfully hack into the banking system in order to steal enough credits for hundreds of butter pies, then gorge himself sick. It's implied that his father is also addicted.
The Stand: Harold loves chocolate Paydays. Larry and Nadine actually follow his trail of wrappers for a while as they head south from Maine.
Jane Austen's Emma: Mr Woodhouse and his elder daughter Isabella both love to have a bowl of gruel regularly.