I'd have to say it was my first computer, a store bought no-name Premier 1600 (286-16Mhz) with a 40meg WD coffee grinder and DOS 4.0. Got it used for $600.
I studied and began to edit BATCH, Config and Autoexec files, DOS commands as well as set up sound cards manually. Of course I don't need that now, but it helped me learn a very satisfying lesson about PC's. I know a lot of "builder" friends who are dead in the water when Windows fails to start.
My current thrill is the super cheap Shuttle AN35N-U board that I use in my personal server. It's dead solid, super clean and a ton of fun for the money.