I say the G4... depending on what you're going to do with it though. It shouldn't be much of a problem running either Tiger Server or Leopard Server (not sure about leopard completely, i'll let someone else pitch in here).
Me pitch 'ere
For Leopard Server a G4 @ 867 MHz is required.
But, that doesn't mean you can't get it running on lower specs, if you don't mind fiddling around and using unsupported setups.
mr1970 has a good point: What are you going to "serve"?
- File & Print services?
- Networking Services? (DNS, DHCP, etc)
-> these two can be done on Mac OS X 10.4 (Tiger) server easily.
- Collaboration services? (Wiki, Blog)
- Podcast Producer?
-> these two really need Mac OS X 10.5 (Leopard) server, preferably Snow Leopard....
- iTunes sharing?
- Simple FileSharing?
-> No server needed at all. Mac OS X "client", can serve simple services on your network easily too.