I am well poised to answer this question because I travel often, and I eat a whole lot of fruit.

This is me in Colombia after a trip to the grocery store to purchase one of every fruit available.

Lets start with the best of the best: Melons of Uzbekistan. Imagine the most mind blowing strawberry you've ever had. Now imagine that strawberry weighs 3 kilos and is just as flavorful. Imagine cutting it into thick slabs, taking huge bites and having its juices run down your chin and neck. That is what it is like to eat a melon in Uzbekistan. No hyperbole, they are just that good. They come in a wide variety, are sold at every market and at stalls by the road, and cost next to nothing.

Other great fruits generally not available in America

Grenadia: The best of the already impressive passionfruit family. Less acidic and more flavorful than its siblings. It is also great fun to peel and eat. Native to Northern Parts of South America.

Gooseberries
I would gobble these up on the dacha in Russia. The flavor and texture is half way between a grape and a kiwi, including the slightly hairy exterior. I've had these in the US but they are usually quite a bit more tart. I also recommend black, red or white currents, which are relatives of the Gooseberry.

Mangosteens
This is the perfect exotic fruit. It reminds me of something you would find on a distant planet in science fiction novel. Like many tropical fruits, you can find some in Florida, but they are native to, and most abundant in Southeast Asia. The crimson rind peels away easily, like moist, thick corrugated board. The stark-white, satsuma-like fruit starts off tart and a touch pulpy, but then immediately melts away into something soft and perfectly sweet. It also has a delightful earthiness, almost like a mushroom.

Alphonso Mango
Last year I was fortunate enough to be in India during the mango season where I, in typical form, purchased one of every variety I could find and had a taste off. The clear winner, by a wide margin, was the Alphonso Mango. It is similar in many ways to the quite lovely Ataulfo (Manilla) mango that I am increasingly seeing in US grocery stores, only better in every way. It is sweeter, more flavorful, more delicate and less astringent than any mango I've ever had. It's just a great goddamn mango.

Sweetsop (Custard Apple)
A ripe custard apple is a special thing. It is almost too soft, falling apart as you try to make sense of how to peel it. The flavor changes depending on how proximate you are to the center or one of its seeds. It is difficult to describe how it tastes, but it certainly has elements of apple. They are native to the tropical Americas, and far superior to its many similar looking cousins.

Important Note
The quality and diversity of fruit in America varies greatly by where you live and shop, but overall, fruit like apples, oranges, grapes, strawberries, blueberries, grapefruits, pineapples, watermelons, mangos, kiwis, etc are among the greatest in the world. Most fruits that look exotic or beautiful are often underwhelming when eaten. These include rambutan, dragon fruit, jackfruit, durian and quite a few others.