I'd get a MacPro with a lot of RAM. This gives me a platform I can expand through third party video i/o cards,esata cards for external RAID boxes, extra internal drives, multiple monitors, etc and will have the longest useable life span.
Your mileage will vary.
What it comes down to is ... Best is a subjective concept.
Best at what?
Absolute speed? Flexibility? Expandability? Life cycle costs? Low cost of entry? or ???
Define your needs, long term plans and budget more closely and what machine to purchase will become obvious.