I agree, there really isn't any 'best' computer.  However, as a general rule, getting a high amount of memory (RAM) is a good idea.  For the most part, that is the first 'bottleneck' that slows things down.

Personally, if I were to be planning to use the computer for a long time, I would get a computer with an "i5" processor.  You definitely could get a lower one (such as an "i3") and be okay, but as the years go on you may notice more lagging, especially if you use Quickbooks or have several programs running at the same time (you could upgrade to an "i7", but I don't think you will see any difference for a tax preparation computer).

I build my own - i3, 16 gb ram, NVMe drive.  After Intel's "8th generation" imho i3 is more than fine. 

Quickbooks, lacerte etc are all single thread programs and really don't benefit at all from higher core processors.  QB even limits its ram usage.  Most of the time its my Chrome tabs using the most resources