I suppose it depends on the software you want to run. Personally I'm nostalgic for those 7xxx series machines I lusted after when I started working in the print industry in the mid 90s. They were way out of the price range of the company I worked for at the time but looked so very sexy. Using a new-world machine (the newer transparent plastic G3 and G4 machines) is much easier today though than tracking down SCSI drives and ADB peripherals. I mean I love my 7300 for example - especially since I can run System 7 on it - but I have everything I need to use that old bird. I used to have an old-world PowerMac G3 machine too, but I gave it away since I have a G3 new world (blue and white) machine and it does everything I need. I also have a B&W iMac G3 233 but it's a nightmare to work on so I might actually sell that as well.
Since my interest is mainly in playing games, a G4 or G5 is not that interesting to me. I know those machines inside out from supporting them every day for years but they were never really much of a target for games developers. With Apple shifting the OS to OS X meant most devs had to rewrite all their tools and while there were lots of good DOS/Windows ports for PPC OS 7/8 machines those dried up mostly with the move to OS X in my experience. Apart from the open source ports of the Quake games etc I don't remember there being much to play on OS X for the first few years. The performance of OS X itself was pretty poor on G4 machines anyway and I also think devs were worried that Apple wouldn't even make it as a company around that time. Then later the switch to Intel also seemed to set the games market back again for a little while - although personally I was more into console games by that time and stopped buying Mac games more or less. G5s are still fairly powerful machines though - I spoke to a friend a few months ago who is still using one for print production work.
Personally I've always wanted to own a Quadra of some kind. The very first Mac I ever used (after having built my own PCs for a few years) was an LC475 but I'd love a Quadra 630 or anything with a full 68040 CPU. The only 68k Mac I own is a IIsi (68030) which is cool and all but very underpowered for anything I'm nostalgic for - although I must say I was impressed by a game called Fool's Errand which works well on that IIsi. None of the Amiga ports are really playable though (Flashback, Lemmings, Worms etc). Surprisingly Myst works fairly well with an external SCSI CD-ROM though.