I am extremely picky about my outdoor gear and often spend weeks, months or even years choosing my kit. I haven't made too many mistakes and most my gear lasts around a decade. Two items have reached the twenty year mark despite very regular use and have become my favourite bits of kit.
Mountain Equipment ultrafleece jacket
Not too thick or bulky (probably much thinner now than it was 20 years ago). Where I go in the mountains it goes too. I think ultrafleece aka karisma fleece was discontinued in 2003, shame.
Montane featherlite smock
Perfect weightless, breathable and windproof layering in any season. Made of pertex, weighs 105g and packs down smaller than an apple. In 2012 I gained a wife and despite my best efforts, lost this item of clothing to Susie. She wears it everywhere and I have had to buy another.