Editorial and personal opinion content:
For a system that will primarily boot Mac OS 9, over the "MDD" series, I personally recommend a Power Macintosh G4 "Digital Audio", "QuickSilver", or "QuickSilver 2003" - those three systems have nearly identical architecture (to the best of my knowledge, the "QuickSilver" is literally the same system board as the Digital Audio, but repainted and with newer CPUs).
These systems (should, again, best of my personal off-the-top-of-my-head knowledge) all share common upgrades. In addition, Mac OS 9 isn't served very well by much more than a gigabyte of memory. There's disagreement as to whether or not it even could use more than a gig, even if it sees it.
In addition, more of the MDDs were sold in dual processor configurations than in previous generations, coupled with better stock video cards and a higher RAM ceiling, the MDD is going to be a good OS X machine where much of the hardware is wasted on OS 9.
In addition, 2, this may or may not matter for your needs, but Mac OS 9 is inconsistent at fan speed control on the MDD machines.
For a system that will dual-boot, the dA/QS/QS'02 systems are still "good enough" (and easily upgradeable) for both 9 and x.
On the go, I usually don't really recommend bothering with G4s for Mac OS 9, but I will say that the most recent go I've had with OS 9 on my 1.0GHz "TiBook" PowerBook G4 has been surprisingly good. I think that the main thing that's made it better is that I installed a relatively fast, good condition 160GB hard disk in it.
That said, again, these are personal opinions and my experience using Mac OS 9 daily ~2004-2007 and again 2017+, day-to-day kinds of stuff isn't meaningfully faster on fast, high-end G4s than it is on, say, reasonably nice G3s.
I don't really have any experience with iMac G4s, eMac G4s, or any iBooks, so I can't personally speak to those machines, other than that any eMac would probably be a really nice OS 9 machine: Those displays are very good, or at least they were when they were just a couple years old, and they have three USB ports. I'd put one on my desk if I found one inexpensively and semi-local (I love road trips, let me know if you're from CA, AZ, NM, NV, UT, or CO and want to unload one!)
My main experiences with iMac G3s are a /233 I had long ago - mine had just 32MB of RAM for most of its life and under that configuration I'd run the original 8.1. With more RAM it should be fine though. I also recently picked up an iMac G3 with a /300 CPU and 256 megs of RAM and I'm anticipating that to be a solid, stable, and quick way to run my personal daily OS 9 workload.
(Most of what I need consists of, Netscape 4.x to browse Macintosh Garden and s7t as well as previewing the vtools web site, running the vtools admin tools, Office 98 or 2001, Stuffit 5.5, diskcopy and toast for media and archive handling, and Dreamweaver MX. I'd do all this on the 6200 or 8600 but DWMX appears to have a hard requirement for the G3 CPU and I haven't gotten my mits on an older copy of DW.)