That would have to be the humble A1200, for the reason that it's so damn expandable. Virtually every single hardware limitation had been overcome with some extremely clever hardware like the Mediator PCI, PPC accelerators, Power Flyer cards that work around the slow on board IDE interface, replacement parallel/serial ports, USB... The list is seemingly endless.
The A1200 is certainly not without it's faults but for me it epitomises everything about the so called community spirit that makes twelve year old Amigas still useable today.
Runners up would have to be the original A500, simply because it was my first Amiga followed by the A3000 because it was the most complete Amiga ever made IMHO.