If you ask many a tea connoisseur, they will probably argue that tea should never have milk or sugar added, or "only the ones that aren't worth drinking." However, beyond that level of snobbery, there are plenty of teas that are enjoyable with a splash of milk or a cube of sugar, and there are many traditions of adding milk, sugar—or both—to tea, spanning all the way from England to Tibet.
When all is said and done, it should just be a matter of taste—if you enjoy milk and sugar in your tea, by all means, add it.
So which teas are best to drink with milk and sugar?
The Bolder the Better
In general, bold, astringent black teas (or red teas, as they are known in China and Taiwan) will be your best bets for adding milk and sugar. They may be tea blends or single-origin teas from India (exempting Darjeeling), Sri Lanka (formerly Ceylon), Indonesia, and parts of Africa and South America. Some green teas (such as Gunpowder green tea) can also benefit from a little-added sugar. However, white teas, oolong teas, pu-erh teas and most green teas are rarely enhanced with added sugar.
Special Milk Teas
As a category, milk tea is enjoyed around the world. These special milk tea recipes include the "builders teas" of England, Ireland, and Scotland, the "chai" teas of India, the yak milk/butter teas of Tibet and Nepal, the "pantyhose milk tea" of Hong Kong, many of the "bubble teas" of Taiwan, the tea latte of North America, and the cream tea found in East Friesland, Germany.
You can prepare these drinks at home using milk tea recipes.
Of those milk teas, many are sweetened. For example, builders tea will have a cube or two (or more) of sugar, Hong Kong's pantyhose milk tea is sweetened with sweetened condensed milk or with added sugar, Indian chai is usually sweetened with jaggery or sugar, and tea lattes are often sweetened with sugar or with simple syrup.
Hold the Milk
There are also many teas which have added sugar and no added milk. Iced tea is often sweetened, especially in the Southeastern United States. Some tea-herb blends are also served sweetened. This includes Moroccan mint tea, which is made from boiled Gunpowder green tea, sugar, and spearmint leaves.
As a side note, sugar is often added to Earl Grey tea in England, but milk is not. It is considered to clash with the citrus flavor of Earl Grey. Yet, strangely, milk is often added to Earl Grey in the U.S.