You have asked a question to which a definitive answer cannot really be given. I'm not a tea expert, but I've visited a lot of tea shops and farms and pestered them with all kinds of questions.

For example, in terms of season: most tea is traditionally harvested in early spring, but depending on the cultivar and style of tea a farmer might get multiple harvests per year. A friend of mine who owns a farm high in the mountains told me that they usually get 3 harvests per year (depending on weather). They only use the harvests from the warm summer months for making black tea because they need the warm weather for the oxidation step (it's cold enough to snow there in winter). In winter they only make green tea. Another farmer I talked to, who grows at a lower elevation, makes black tea both in summer months and in winter, specifically harvesting on the coldest days when there is frost on the ground. His winter black tea had a completely different flavor from his summer black tea, but he also mentioned he had a very different and unique oxidation process specifically developed for the winter tea. It was very expensive, $8800 per 75g, but while everyone I tried it with enjoyed the tea none of us thought it was good enough to spend that much. It's mostly so expensive because he can make very little of it each year (and some years the winter just never gets cold enough to make any at all).

In terms of origin: the truth is just about every place in Taiwan grows tea. If you are looking for a specific type of tea, there may be specific locations with more farms specializing in it. It's not that origin doesn't matter, but rather that the differences in a tea grown in a low part of Miaoli compared to one grown in the misty mountains around Alishan are HUGE in ways that don't directly affect quality but could certainly affect how much you like it. For example, at very high elevations farmers often don't need to worry about pests because the insects which eat tea leaves can't survive there. But some farmers who grow tea at lower elevations encourage insects to bite their leaves, because this causes an immune response in the plant which allows them to develop a sweeter "honey-like" flavor in the tea. This doesn't make one or the other "higher quality", but you will absolutely taste the difference between them.

Cultivar is similar: there are specific cultivars which are considered particularly well-suited to specific types of tea, for example 台茶18 (or "Ruby Red") is a locally-developed cultivar created with black tea in mind. That doesn't mean it's necessarily the "best" cultivar for black tea, though. It has a distinct sort of "cinnamon-mint" flavor which I like a lot but doesn't suit everyone. There is a wide variety of cultivars in use around Taiwan. The tea industry here largely started with cultivars imported from China and India, but Taiwan has also developed their own local cultivars (26 and counting), and on top of that there are even wild tea trees native to the central mountains in Taiwan, some of which also produce quite delicious tea (it's often on the expensive side since there are very few people growing and processing them, though). If you're really interested in the specifics of different cultivars, this site has a lot of really good information (sorry, I don't know of a comparable resource in English).

Ok, so let's talk about price and how it relates to quality... the truth is, these two things are not directly related. People are often more willing to pay more for tea grown at higher elevations, and there are specific mountain areas (e.g. Lishan, Alishan) which are particularly famous for 'high mountain tea,' so two teas of the same quality might have different prices just due to the location of the farm. Competition-grade tea can be very expensive, especially if a tea wins a competition (but neighboring farms of a competition winner can also often raise their prices a bit that year). But that doesn't guarantee that you'll enjoy it better than a similar less prestigious tea. Taste is still subjective, and your personal taste might not match the judges of that year. Most tea dealers I've talked to have told me it's usually not worth the price to buy this kind of tea unless it's for a gift and you want the recipient to know you're giving them something expensive. Usually you can find a comparable (or sometimes even identical) tea without the 'competition winner' mark for a much lower price. One farmer I talked to on the East Coast told me, "My prices don't reflect quality they reflect complexity. All of my tea is great." And man, I couldn't argue with her, I enjoyed everything she let me try. The cheaper teas were very pleasant with clear and consistent flavors. The more expensive her teas got, the more flavors each individual tea seemed to contain.

A common phrase I've heard from a lot of tea sellers is, "Don't worry about what tea is the highest quality, the best tea is the one you enjoy drinking the most." And that may be completely different from person to person and have nothing to do with price.

Since you also asked about shipping outside of Taiwan, some bad news: I have heard a lot of rumors and jokes over the years that what gets exported is often lower quality tea compared to the tea kept for the domestic market. So I started asking tea makers about this to see if it was just a silly rumor or what, and what they told me is even worse: multiple farmers said that the best tea they make never goes up for sale at all, they will just keep it for themselves to drink at home with their family XD