I don't leave home without one of my Xped sleeping pads (summer or winter version) b/c it is the most comfortable I have personally found. It allows a good night rest for those 20+ mile days.
I like my MSR Pocket Rocket 2. So small, but effective - even simmers.
Second, I have a hard time leaving my GSI Java Press behind.
My favorite piece of backpacking gear is my air mattress with 3" of comfort underneath me. I sleep so much better with that cushioning.
For me I never leave without at least 50' of parachute cord. There are so many different uses and every time I am out backpacking I find a new reason to need it.
Treking poles! They help so much when I start to get tired or have big slopes to go up or down.
Similar idea: buff. I prefer the merino wool for backpacking.
yup - wife and I hiked down Grand Canyon the Colorado river from North Rim (via Tapeats Creek) - I had an Xped, she had a 3/4 Thermarest. Come to pass, she snuggled up with me all night on my Xped several nights in a row. We both own full size Thermarests (decades old, and in perfect working order)- but they were too heavy for us oldies to carry.
No favorites as such. My oldest piece of gear I still use are a pair of REI mid weight synthetic long johns I bought in the mid eighties and have use variously for sailing, skiing and backpacking. My activities have been intermittent so the longevity is not quite as impressive as it seems. My 10 year old Gregory Baltoro 70 pack is still very comfortable for carrying 7 days of food in a bear can. At 5.6# it is heavy by today's standards but it's the last thing I would change out. Weight distribution is as important as absolute weight. I was very pleased with my Exoffico give and go briefs, Darn Tough wool blend boot socks and Glacier Glove Ascention Bay sun gloves all new this year.. Also new this year, my Big Agnes Tiger Wall 2UL worked well in Desolation Wilderness and on the JMT for both 1 and 2 people.A slightly more unusual piece of gear that worked well and I will continue to take is a Brondell GoSpa personal bidet.