The fastest that works without hacks is the Mirrored Drive Door G4 range of towers. It is tricky, because the FW800 also have mirrored doors, and look pretty much identical. But they are easy enough to tell apart from the rear, as MDD have only the two FW400 ports, and the FW800 has those plus one FW800 port. The fastest laptop is the 1GHz PowerBook Titanium, or TiBook. I have both, and the MDD/FW800 are quite a bit faster, in my experience. One thing to look out for is that there IIRC was a "budget" MDD with a slower 133 MHz bus and 867MHz CPU. The rest of them had a 166/167 MHz bus. Many swear by the earlier Quicksilver Macs also, as being fast and stable, but I've never used one.
Personally, I would call the FW800 with DieHard's install CD for unsupported systems a "sure thing". The installers here are all great. The MDD are fast, but they are also very picky about what version of OS 9 they run. Because OS 9 was losing support, Apple made special versions of OS 9 just for these machines. The ones in the installers here are the latest and best available.
These old Macs do have a reputation for reliability, but in my experience how low-maintenance they are depends upon how much use they see. Most users have been casual, or doing light-duty work. It's not unlike having a vintage race car! Easy Sunday drives don't tax it much, but doing hardcore multitrack sequencing, video/multimedia capture and encoding, etc and it takes a bit of effort to set it up well and keep it going. I have had Macs for nearly twenty years and I am still learning. More than ever, really.