My first DAC was some cheapo optical to RCA converter to get audio out of my bare-bones plasma screen into my circa 1977 Kenwood integrated. It worked well enough for TV Audio.
Then I bought a Cambridge DAC200 for use in live sound production, and it made enough of a difference that I could hear the improvement in my computer speakers.
When I got serious about a system for my office I started with a Parasound DAC, but it was unreliable and eventually blew up one of my ELAC speakers, so I picked up a Pro-Ject Pre Box S2 that sounded better all around, better bass control, longer tails, etc., but the big upgrade was a DAC3 from Benchmark. I'd say the biggest improvement is in SN ratio, it's dead quiet and since it's the HGC version I can run the output level at full studio-level (0db) into my AHB2 amp. I'm very satisfied with this combo, feeding ATC monitors.