Footwear – comfortable, light and supportive boots (good running shoes are an option if you have sturdy ankles and a very light pack)
Rain jacket – Gore-Tex and similar materials are good for certain conditions but cheaper options usually work fine
Shorts or convertible pants - fast drying synthetic (non cotton usually nylon or polyester)
T-shirt - cotton or synthetic
Thermal underwear - synthetic tops and bottoms
Insulating layer - fleece jacket or wool sweater
Socks - wool, synthetic or a combination
Sock liners – optional, helps some people prevent blisters
Rain pants – lightweight non-coated nylon doubles in summer as bug pants
Hat and gloves – mountaintops can be chilly even in the summertime
Gaiters - usually low style helps keep pebbles and dirt out of boots
Good footwear is the key. Whether you chose boots or running shoes they need to fit and they need to be broken in. Nothing short of a broken bone, will slow you down more than improper footwear. Footwear also wears down over time. When I hiked the PCT I went through three pairs of light hiking boots. I fellow hiker went through 6 pairs of high quality running shoes. That was for 2600 miles of hiking. I ran into several folks who had pushed their shoes beyond their useful life and had ended up with stress fractures because of it.
Heavy boots last longer but I only know of a handful of folks who have made a whole long distance trail on one pair of boots. Heavy boots also require long periods to break in. You feet will break before your boots break in if you do not start off slow and steady. In a hot climate, full leather boots can give you heat blisters.
Properly fitting lightweight boots and running shoes are broken in out of the box. They also break down much quicker, as mentioned above.
Mid weight boots offer a good compromise.
For much of the Western US, I prefer breathable boots with a full length nylon shank. Kind of a cross between a lightweight and mid weight hiking boot. But these are very hard to find. In the winter I use mid weight leather boots for their water proofness on the snow.
Another issue, is that feet change as you hike. The hiker who went through six pairs of running shoes had his foot grow three sizes due to his arch falling from all those steps with a pack on. I have the advantage/disadvantage of flat feet, so my foot size barely changed.