Technically speaking, Housman originally labeled "Loveliest of Trees" with a "II." It was, after all, the second poem in a volume called A Shropshire Lad (1896). A lot of times when a poem becomes popular, but doesn't have an actual title, people just use the first words of the poem as the title. We're gonna go ahead and roll with that.
Now, when you see a title like "Loveliest of Trees," you can assume one of two things: that it's about a particular tree that the speaker thinks is just the best ever (like that one in my backyard), or it's about a specific species (such as the cherry tree) that he feels the same way about. Once we start digging into the poem, it's clear that the speaker is not talking about one specific tree, but a whole species: the cherry tree.
We've looked at some pictures of blossoming cherry trees, like these, and we have to agree that it is one of the loveliest of trees. This isn't just a poem about how pretty cherry trees are however, or rather, that's not the only the thing the speaker means by "loveliest."
The cherry tree is the "loveliest of trees" because it is that type of tree that has made the speaker realize that the world is full of many beautiful things—many, many "blooms." The cherry tree is so beautiful, and it makes the speaker think about all the beautiful things in the world, and all the things he wants to do before he dies (he doesn't say this directly, but this is what he means).
At the same time, the tree's blossoms also remind him of snow, of the winter to come and of his own eventual death. Since his time is limited, he decides that he better get with it and start enjoying life and the beauty it has to offer. In two ways, then, the cherry tree is the loveliest—not just for their physical beauty but also because it causes our speaker's stunning realization and renewed commitment to life.