I have a lot of favorites and I use all of these nearly daily for various routine tasks.
The Wallstreet because ADB, Serial, and 2 card slots. Plus the ability to Run 8.6-10.4. I use one exclusively as a print server for my Stylewriter (9.2.2 & 10.3), and another as a go to 8.6 machine. Connects to the Newton without so much as a thought. The ability to swap batteries and media drives is also a huge plus. This is probably the most versatile computer in the arsenal. Starcraft anyone?
iBook G3 466SE iBook. This is my go to G3 machine for running 9.2.2 and 10.4. With an upgraded SVGA LCD, this computer is gold.
PowerMac G4 1GHz DVI: when 9.2.2 and 10.5 are essential I ALWAYS grab this one. The ability to pull disk images from the web quickly with 1GB ethernet, then boot into 9 and burn a Classic bootable disk. You can't beat this work horse.
17" PBG4: this is my go to portable studio. I like running Logic in 10.5 on this guy.
12" PBG4: in my opinion, this was the best portable Apple ever built. Yes, I do say better than the Duo. It's agile, no frills, but has connectivity that's nearly standard to it's 15" and 17" cousins. I use this as a strict 10.5 machine and I access Plex and my main iTunes/iPod library from here.
20" iMac G4: I love the G4 iMac design. With the display able to move towards you, the experience of using this computer is spectacular. I use it mainly for Sibelius notation work. If you need to check small details, forget zoom, just move the display closer like a picture or a book. Genius.
PMG5 Quad: I used to build an Xgrid with about 15 various computers that could run 10.3/10.4 for a few weeks every year and do mass video encoding to support 603, G3, and G4 machines on my Plex server and AFP shares. But with a G5 Quad, it'll do it by itself and hardly strain it's eyes. I dual boot 10.4 and 10.5 server and run lots of diagnostic netboots and netinstall images so when things go down I can get them back up in a hurry.
Finally, a more modern one: 17" MacBook Pro. It's a mobile desktop for all of my modern tasks. ExpressCard, FW800, 3 USB ports, Thunderbolt (on 2011 models).
Oh man, so many more to choose from. I probably would have also said my hot rodded cube too, but it's currently not in use do to space constrictions.