In Stardew Valley, there are just about two dozen different people you can give gifts to, and most of them have their own tastes. Some people love some things, while others do not. If you're looking to level up your friendship with someone, though, you'll definitely want to focus on giving them the gifts they love, as you'll receive more friendship points than something they just like.
Aside from each person's individual tastes, there are five gifts that are universally-loved, often called the "universal loves" by players. Basically every character loves to receive these gifts, with only a couple of exceptions. These items are very rare, though, so your chances of having them sitting around available to gift to someone are slim. However, here's where you can find all of them if you want to give them to that special someone.
The Golden Pumpkin
The Golden Pumpkin is a fun item, though it doesn't really serve many important functions. The most common way to find one is at the annual Spirit's Eve event, which takes place on the night Fall 27. The event has a maze that you can make your way through for a prize, which is the Golden Pumpkin. However, you can also find them (rarely) from cracking open Artifact Troves.
You can find one every year at the Spirit's Eve festival, not just your first year, which is nice.
Prismatic Shard
Prismatic Shards are one of the most sought-after items in the game, as they are used for a ton of different things, from getting the Galaxy Sword, to enchanting weapons at the Volcano Forge, and more. Additionally, they're also a gift that (almost) everyone loves to receive. The one exception to loving this rainbow mineral is Haley, who hates them for some reason.
You can find Prismatic Shards in a number of ways, but some of the most common are from fishing treasure chests, from Mystic Stones in the Mines and Skull Cavern, from Omni Geodes, and from the Statue of True Perfection, but there are methods as well.
Rabbit's Foot
The Stardew Valley rabbits seem to have a strange magical method of regrowing their feet or something, because they can drop a Rabbit's Foot an unlimited number of times without ever seeming to not have all of their feet, visually. Regardless of this strange phenomenon, Rabbit's Feet are an almost-universally-loved gift, with only Penny hating them.
In order to get a rabbit to drop a Rabbit's Foot, you'll need to befriend it and make sure it's comfortable and fed. If your friendship level is high enough with a rabbit, it has a chance to drop one based on your daily luck. You can also purchase them from the Traveling Cart or, rarely, see one drop from a Serpent in the Skull Cavern.
Magic Rock Candy
The Magic Rock Candy is arguably one of the hardest items on this list to obtain. However, get your hands on one, and anyone you give it to will love it, without exception. You can trade in three Prismatic Shards to the Desert Trader on Thursdays for a Magic Rock Candy, you'll get one for donating 90 items to the Museum, or, if you're very lucky, one might drop from a Haunted Skull.
Even though it's a universal love, the Magic Rock Candy is arguably wasted if you give it away. It's one of the best food items to consume when mining, as it'll give you several buffs: +2 Mining, +1 Speed, +5 Defense, +5 Attack, and a whopping +5 Luck, as well as healing 225 health and restoring 500 energy.
Pearl
There a few different ways to get your hands on a Pearl, and if you do, you can give them to anyone, as they're universally-loved. You can find them inside Artifact Troves, from the Mines after the Shrine of Challenge is activated, in a Blobfish pond that has at least nine Blobfish, from Willy after filling a fish pond with Crabs for the first time, and from solving Journal Scrap #6.
However, the first method for obtaining one that many people will find is by solving the puzzle aboard the Mermaid Boat at the Night market. Unfortunately, this method can only be used once per save file. A mermaid on the boat will perform a song for you. Afterward, you can click on the clamshells to play musical notes, and if you do it in the right order, you'll receive a pearl. You can find the solution to the puzzle in Secret Note #15, but you can also just figure it out by listening to the song.