I don't know if I really have a single favorite purchase, but one that comes to mind is the first ice axe I bought myself (in contrast to one my dad bought) in 1978. It's still stashed away somewhere under all my gear and I haven't used it since 1995 or so, so I don't even recall the make. Even in my old pretender-mountaineer days I didn't use snow and ice gear that much, so it hasn't seen much use. However, the circumstances surrounding its purchase and one particular self arrest I made with it, are what gives it sentimental value to me.
First of all, the circumstances of its purchase. I was an undergraduate in the dorms at Berkeley. As of 1978, I recall there were only two REI's in the Bay Area, the first and biggest of the Bay Area REI's (Berkeley) and the 2nd one in Sunnyvale. My dad had been mail ordering from them as far back as the 60's. I remember that an REI sale was a huge event in those days drawing huge crowds. For those of you who know the Berkeley store at Gilman and San Pablo, the sale lines in the 70's would form prior to door opening and reach to Gilman by the time they opened the door. In any case I had decided I would go to an REI sale (was this annual back then? don't know, but pretty sure it was in the fall). I rode my clunky 3 speed bike from the dorms to the Berkeley REI. I stood in a pretty long line and got in at door opening. I decided that this particular ice axe on sale was just what I needed--I recall the price was something like $29.88 (.88 was the REI sales suffix price back in those years, apparently) which was about what I paid for my basketball shoes back then (basketball shoes have inflated much more than ice axes in the years since then).
In any case, in most other cities, I figured I'd attract some strange looks, or worse, if I rode through the streets on my bike carrying an ice axe. In Berkeley, I figured nobody would think this was odd or abnormal. I was right. The only comments anybody made (from cars or as pedestrians) were pretty much the same: a few separate folks simply shouted out "nice ice axe!"
The next spring the ice axe made its debut on a spring (early May) ascent of Mt Brewer. On the descent I tried a very ill advised glissade down the headwall on the E Ridge, fell, and took off flying down the steep but very soft spring snow. Initially the tried and true self arrest didn't seem to work and I was pretty stressed as my point slices through the snow like a hot knife through butter. Eventually the self arrest was successful with about 100 feet left before the the snowslope plunged over a 500 foot cliff. I had made self arrests before (and since) when the consequences of failure may have been moderate injury, but the Mt Brewer fall was a true do or die self arrest, so this particular ice axe is my sentimental favorite.
Most recent favorite purchase: Without a doubt my Asolo TPS 520 boots. These were the replacement for whatever I had as my designated death march backpacking boots. They have the combination of protection (from those side hits from corners of rocks etc.) I need for the off trail rocky stuff I do so much, plus terrific balance (no boot I've ever had gave me support like this on those long steep sidehill moves), yet much lighter weight than the old tank-like boots I wore on death marches past, and a much higher degree of comfort. As much outdoor stuff as I do, I have tended to have traditionally had three "grades" of boots: my most rugged stuff for death marches, my "medium" stuff for other Sierran hikes, and my lowest grade stuff for low altitude field work and leading my students on field classes. It is true that some of the field areas I have my students work in are quite rugged, but having somewhat flimsy boots acts as an added handicap that slows me down some so I don't have to wait as long for most of my students to catch up with me when I lead them between outcrops (my crummy boots kill me when I sidehill, for example). However, because I haven't been going on death march backpack trips lately, I've been using my Asolos on somewhat easier Sierra hikes as well as using them for my most rugged personal field geology work (usually in the Coast Ranges). It was in the latter that they've really made a huge difference, particularly in some nasty areas where I have to deal with thick brush, loose class 3, funky sharp edged talus, and steep "baked dirt" covered with gravel, all on the same slope.