It's pretty cool, really. I believe the MP4,1/5,1 hold the record in terms of sheer number of OS X/macOS versions they're compatible with. The 4,1 can even install Leopard, but I'm not sure if it's blocked from installing on the 5,1 (and 4,1s flashed to 5,1). But even if so, Snow Leopard to Mojave is a whopping nine versions. And call me an optimist but I think based on past history that chances are high next year's release will have the same compatibility list as Mojave. That would be ten versions!
Leopard will boot fine on a MP5,1 as long as you have Nehalem CPUs and not Westmere CPUs.
as for the system which can boot the most OSs, that would have to go to something like a Early 08 MBP4,1 which can boot 10.4.11 all the way to Mojave with some hacking.
(an iMac7,1 can also do the same if you upgrade the CPU to a Penryn one)
and in theory you could do the same with a MP3,1 but I have not seen anyone boot Tiger one however I suspect it should be possible as I saw a user here boot tiger on an Xserve2,1 which shares the same HW platform as the MP3,1.
Is Snow Leopard compatible with the new firmware?
the answer to that is literally right above you