I say Plus (1.1-7.5.5, 160MB SCSI Disk) then a 7600 (7.5.3-10.4, XPostFacto to http://www.jump-ing.com/?en/Projects/Tiger_on_7600 10.4), then a Mac Pro 1,1 (2006, 10.4-10.10 with hacking).
With this setup (which requires LOTS of hacking, specially for Tiger on 7600) you can go for 30 years of OSes on 3 Macs. (But, a Plus under 7.5.x and a 7600 under 10.x will both be very slow. I suggest System 7.0.1 (with a hard disk) for the Plus, Mac OS 7.6.1 for the 7600, and 10.10 for the Mac Pro. OS 8 might be good for the 7600, but I love 7.5.x/7.6.x)
With this setup (which requires LOTS of hacking, specially for Tiger on 7600) you can go for 30 years of OSes on 3 Macs. (But, a Plus under 7.5.x and a 7600 under 10.x will both be very slow. I suggest System 7.0.1 (with a hard disk) for the Plus, Mac OS 7.6.1 for the 7600, and 10.10 for the Mac Pro. OS 8 might be good for the 7600, but I love 7.5.x/7.6.x)