Narrowing down this list to three is actually rather difficult. Each trip has conditions and situations that make one piece stand out relative to the others. That said, I'll follow someone else's lead and post some runners-up as well.
1) Montane featherweight smock and pants. I put these together because I think of the suit as one piece of gear. I treat them with Nikwax TX-Direct Wash-in about every 10 washings, and they're my first line of defense against almost any change in weather or insect pressure. I don't know how I could live again without these (or something very much like them) on the trail.
2) Moonlight Pack by Six Moon Designs. I did have some problems with the foam in the straps getting compressed and not recovering, but after Ron replaced it, it's been great. Just the right amount of volume for my gear, and very durable. I own a couple of other lightweight packs, but this is the one I'll grab by default when I'm most uncertain of conditions.
3) Watch, Suunto Vector. This watch, altimeter, and compass in one is sort of a one-stop telemetry station. It's often the only electronics I take with me. Actually learning all of the fancy compass modes has made it more useful than one might think on first impression.
A) Bozeman Mountain Works Spin Poncho. It was tough deciding between this poncho and my Equinox extension-poncho. I like the extra length and the off-center hood position of the Equinox -- it covers my pack better and provides more coverage as a shelter, but I like the lesser weight of the spin-poncho more.
B) Platypus Big-Zip Hoser, 2 liter. The lightest hydration system around for the volume. I cant tell that it imparts any taste to the water. I've never had one leak -- I've put one into almost every one of my packs.
C) Princeton Eclipse 2 flashlight. This LED flashlight and headlamp is terribly versatile. One clip attachment lets me use it as a head lamp, anotther lets me hang it from a lanyard. It has three brighness settings and a flashing mode. It weighs a little more than a Photon Micro-light, but the couple of tenths of an ounce are worth it for the added utility.
You may notice I didn't mention any sleeping gear. I really liked my Nunatak Ghost Blanket, which I just got rid of. I have a couple of home-built polarguard quilts, and a BMW Arc-X (which I haven't actually tried in the wids yet). They're all excellent for different situations -- I just couldn't decide among them.