Easy. My Abit BX6-r2 i440BX mobo, hands-down. Most stable, compatible, mobo *ever*. Also rather ahead of it's time, in terms of features and voltage/multi support for Slot-1 CPUs. I'm sure that some people are probably using one of them to this day. I would be too, had I not shorted out the CPU VRM circuitry from a bad Tualatin mod job. Oops. :|
Second to that would probably be my Yamaha 6416S CD-RW drive. Built like a tank, and was still working when I sold it several years later.
Oh yeah, can't forget my IBM Trackpoint II keyboard, PS/2 interface, wonderful "Model M" clicky-ness, and a trackpoint widget to boot!
I wish they would build modern PC parts like they did back in the day. None of the new mobos/keyboard/drives really impress me anymore.
Oh yeah, can't forget my old dependable USR Courier v.Everything external. Stealth-black, with many wonderful blinkenlights. Still using that one, right now in fact.