Well, as far as my main dream machine of all time, I'd have to say the Quadra 900/950 because they were far and away the best desktop systems of their era. Speed wise they were only matched by the low end workstation market (systems like the NeXTstation, IRIS Indigo and SPARCstation LX). At a time when PCs where maxing out at 16 MB of memory, these Quadras could hold 256 MB (both the IRIS Indigo and SPARCstation LX topped out at 96 MB). Where PCs were limited to between 500 MB to 2 GB drives, there was room for three internal SCSI drives with almost no limit on size (other than what you could find at the time). There potential was just staggering back then.
Coming in a close second would be the PowerBook Duo series. The idea of having one system without compromises, that could serve as both a desktop and a subnotebook was incredible. People are thrilled with the MacBook Air's 3 pound weight today, but the Duos were already around 4 pounds back in the early 1990s.
And they were quite fast for the period too... The 210, 230, 250 and 270c were all around the speed of the Macintosh IIci, the 280 and 280c were about the same speed as the Quadra 900/950 series, and the 2300c was about the same speed as the Power Macintosh 8100/80. When paired up with the Duo Dock, you had a lot of expansion options too. Two Nubus slots, built-in video and room for an internal hard drive made for some great desktop configurations.