Denali was formed 56 millions years ago by the process most mountains of the world are formed - tectonic uplift. However, unlike other tall mountain ranges such as the Andes and Himalayas, Denali is not located where two major plates collide. Rather, it is further inland, along an intracontinental fault named the Denali Fault. When the Pacific Plate is subducted under the North American Plate as it is along the Alaskan coast, the stress extends far inland, compressing and folding the land. So even though it is not located as close to the plate boundary as the Cascades or Sierra Nevada, Denali is still a direct result of the collision of the Pacific and North America Plates.
Denali is made of granite, which is very resistant to weathering. On the other hand, many of the other peaks surrounding Denali in the Alaska Range are much older and are made of sedimentary rocks such as shale, sandstone, and limestone, all of which are far less durable than granite. As a result, neighboring peaks have been weathered down while Denali stands tall above them all. At approximately 18,000 feet, Denali has the highest base-to-peak rise of any mountain located entirely above sea level. That's higher than Kilimanjaro, and Mt. Everest doesn't even come close.
Seeing the summit is a very rare occasion - it is usually shrouded in clouds. Thank you Tessa Czech for the fantastic picture!