The A4000 is probably the most expandable Amiga of the bunch.
So if you want a "general-use ultimate Amiga" you'd want to get an A4000 with a tower kit of some sort (one with a 7-slot Zorro III busboard, maybe a Mediator for its PCI slots...) and a Cyberstorm PPC. Preferably with a 604e/233 and an 060/60 (or a real full non-LC 060/66 or 060/75 if you can find one -- they're rare).
As an aside, I'd pay good money for a card with Prometheus logic that fit the Mediator 4000's backplane. Open PCI spec that fully works with OS 4.0.