There are (at least) three correct answers to your question:
1. The tallest known tree is currently a Coast Redwood (Sequoias empervirens) individual living in northern California. It is known as the Stratosphere Giant, and it is about 370 feet tall! This species lives only in a small strip of coastal land in southern Oregon and northern California,and it's estimated that only 4% of the original number of trees remain today.
2. If you also consider volume and circumference, the General Sherman tree is the biggest single tree. This is a Giant Sequoia (Sequoiadendron giganteum) tree that lives in Sequoia National Park. General Sherman is only about 275 feet tall (only!), but it is 103 feet in circumference at the ground! It's first branch is 130 feet off the ground, and this branch is bigger than most whole trees. Fortunately, Giant Sequoias are not endangered, but they live only in isolated groves inside a 260 mile band on the western slope of the Sierra Nevada Mountains in California.
3. The real winner, though, is the Quaking Aspen (Populus tremuloides). This normal-looking tree sends up new shoots (new baby trees) from its roots, but these new trees are still part of the parent tree. They're the same individual, just a new body part. In Colorado, there's a single Quaking Aspen individual that covers over 200 acres! This plant weighs over 6,600 tons! It looks like a small forest of separate trees, but scientists have confirmed with DNA analysis that they're all the same plant. Now that's big!

So you can see that, like most scientific questions, there are a number of correct answers to your question.